Isaiah Thomas is a problem in the 4th quarter.

In the Celtics' 113-109 victory over the Detroit Pistons on Monday, Thomas scored 24 points on 6-of-9 shooting in the final frame. He was incendiary in money time. Watch toward the end of this video how he manipulates the smallest gaps of space before unleashing his uncanny ability to shoot at odd angles to get the bucket:

Thomas does this kind of thing consistently, rendering his height disadvantage all but moot as he has emerged as one of the premier scorers in the league and a big-time playmaker for a playoff team. He's an All-Star reserve and many feel he was snubbed as a starter, players, coaches and analysts roundly praise his game and guts, and there's even some buzz around him for MVP.

Much of this, again, is on account of Thomas' fourth-quarter heroics. They call him the King of the Fourth for a reason, as he averages 10 points per game in the 4th quarter, shooting 47 percent from the field and 42 percent from 3-point range. The man is a demon in the final frame.

He's not just getting garbage time buckets, either. Thomas is second among key players in clutch-time scoring, averaging five points per contest when the game is within five points in the final five minutes. The Celtics have a ridiculous 122.7 offensive rating when Thomas is on the floor in clutch times. They are unstoppable.

So Thomas is the King in the Fourth, the Celtics are 30-18, and everything's right as rain.


Here's the thing.

In that win over the Pistons Monday, the Celtics were outscored by five in the 4th quarter with Thomas on the floor. Let me say that again: Despite Thomas scoring 24 points on his own in the 4th, Boston was outscored and needed clutch free throws from Thomas late just to survive. It was not a solitary event.

In fact, the Celtics have a negative net rating with Isaiah Thomas on the floor in the fourth quarter. That means that, per possession, they are actually outscored by their opponent. That seems impossible given everything I just wrote, but it's true. Check the stats:

Celtics in the 4th quarter Minutes Offensive Rating Defensive Rating Net Rating
Thomas on-court 321 120.6 122.3 Minus-1.7
Thomas off-court 207 116 95.4 Plus-20.6

For the season overall, the Celtics are plus-2.8 with Thomas on the floor but plus-3.4 with him on the bench, and that comes after Thomas' last 45 days of play where he's been completely en fuego. For much of the season, they were worse overall with Thomas on the floor, which makes you wonder how this will look if Thomas cools off at all.

It's not about his offense, of course.

It's the other end.

Listen, a 122.3 defensive rating in the 4th quarter is just shockingly horrendous. Whatever you think about Boston, they have been bad on that end this season. The Celtics are currently 20th in defense this year.

Now, as it was with Kawhi Leonard, it's here that we have to start with how you need to understand what that fourth-quarter stat means. It doesn't mean that the Celtics' defense is bad because of Thomas, necessarily. It means that with Thomas on the floor, the defense is bad. There can be, and are, a lot of reasons for this, and when you add them all up, you have to at least wonder if the offense, as incredible as it is, isn't worth the tradeoff.

That's a strong assertion, and I'm not making it. I'm simply looking into it.

Let's start here. Is it another teammate dragging Thomas down?

A tiny elephant in the room

Let's look at how Thomas' teammates' fourth-quarter team ratings look with and without him.

Thomas on-court, defensive rating Thomas off-court, defensive rating Differential (red means defense is worse with Thomas on-court)
Al Horford 128.4 96.2 32.2
Avery Bradley 121.5 100.7 20.8
Jae Crowder123.4 84.0 39.4
Marcus Smart 124.4 90.3 34.4
Amir Johnson 118.5 85.5 33
Kelly Olynyk 120.8 93.9 27.8
Jonas Jerebko 113.1 105 8.1

That's right. Every single player's defensive rating gets significantly worse with Thomas on the floor. Maybe more notably, look at that second column. Those are all great defensive metrics. Boston's defense, both individually and collectively, is terrific when Thomas isn't on the court. When he's on the court, they are cheese being melted in a microwave for too long, starting to burn and smell bad.

There's been a lot of talk recently whenever the Celtics have struggled defensively that it's about Avery Bradley's defense. One, as Celtics Blog has brilliantly broken down, Bradley's capable of being a brilliant defender but had a few rough stints. Even then, you see above that the Celtics defense is still great when he's on the floor... and Thomas is not.

I'll spare you the full table, but if you're like me, you thought "OK, but the offense is obviously way better with Thomas, too, right? Well... not so much.

In the 4th quarter, everyone's defense falls off a cliff with Thomas on, and their offense doesn't really improve considerably. The Celtics are still lights out offensively without him.

There's a lot of evidence here that Thomas is the isolated problem. Except that Thomas' man only accounts for 106 of the 1,184 points Boston has surrendered in the 4th quarter as of last Friday.

One more note on these numbers: using per-100-possession data accounts for pace and gives you a more accurate view of what's going on. But it also means that you're not accounting for small changes in flow. By that I mean that while the Celtics are a minus-1.27 per 100 possessions in net points with Thomas on the floor in the 4th, they're a raw plus-12 over 356 minutes and 42 games in the 4th with Thomas on the floor. That's not a wide margin of victory per game, but they have outscored their opponent -- and that's the point, isn't it?

Why are the two numbers different? Offense-defense substitutions. Thomas is on the floor for more offensive possessions than defensive ones. Which could mean that if he were on the court for those, the Celtics' good defense in such situations would reflect on him. Or, it could mean that they're getting those stops because Thomas is not on the floor. It's impossible to know because it's a counter-factual, but it should be noted when we talk about these things. Brad Stevens is clearly doing a good job managing these situations possession to possession.

I realize this is a lot of statistical analysis that might fly over the head of a lot of people, and to some degree, one could make the argument that if you dig deep enough you could find a problem with anything. At least in this this case, I would disagree. As great as Thomas has been -- and he has been great -- he does present a problem for the Celtics in that he is very clearly their best offensive player, particularly in money time, and yet, as currently constructed, they are not equipped to compensate for his defensive shortcomings, especially not against the elite teams.

Where, specifically, are the problems?

First, it's never one thing when the metrics are this messy. It's not like every single bucket scored is because Thomas is unable or unwilling to defend, and that's important in determining where to go from here. Here's what this data doesn't tell us:

  • It doesn't tell us that Isaiah Thomas is a bad NBA player who should be benched.
  • It doesn't tell us that the Celtics should bench Isaiah Thomas in the fourth.
  • It doesn't tell us that Thomas is the reason they are bad defensively.

When you bring up these stats, that's what people tend to assume because Thomas is the variable you can identify as the one concurrent element in the bad situations, and the missing one in the good. But while it's certainly more complicated than any of those things, a lot of the base level explanations you would expect to reveal themselves here don't add up.

For example, the easy answer on these things is to blame the rebounding. The Celtics, after all, are dead last in defensive rebound percentage this season, meaning they grab the lowest amount of opponent misses. While Thomas is never going to be a good rebounder at 5-foot-9, that's not his responsibility. Plus, the Celtics are giving up 14.1 second-chance points per game this season, which is basically identical to the 14.2 second-chance points per game they surrendered last year, when they had a top-five defense.

That is not what has given them issues.

A more complete answer comes with the lineups that they use. In early December, noted that the most commonly used lineup for the Celtics is a three-guard lineup with Thomas, Bradley and Smart, and it gets absolutely shredded defensively.

Part of that has nothing to do with Thomas, it's that the Celtics are going super small in that situation. It's one thing to play three guards together. It's another to play with three guards all under 6-4. It's an even bigger issue to play three guards under 6-4 with Al Horford at center. Horford was great at center in Atlanta, in part because he played with Paul Millsap, and the Hawks' defense was long and athletic. They would also hedge with Horford and play more aggressively. In Boston, he's typically sagging on pick and rolls, which results in situations like this:

Even then, though, Horford has helped contain in the pick and roll. Boston's defensive numbers are fine when it comes to the pick and roll. It's the pass-outs that hurt them. Boston is 17th in defending pass-outs from the pick and roll. However... Horford is in the 13th percentile in isolation defense. Much of that comes off switches from the pick and roll. Putting Horford and Thomas together on a pick-and-roll coverage is not great.

If you want to see where Thomas' negative impact is felt on the defensive end, it's not a surprise. It's his size.

Transition's an easy one to find this in. Boston has given up a high number of buckets in the fourth quarter on transition scores. When Thomas is the one back, there's just not much he can do:

This one might be my favorite. Here, Thomas doesn't even try to stop the ball. He instead grabs Noah Vonleh, and then flops to try and draw an off-ball foul. It's a nifty trick if it had worked:

Here C.J. Miles, not exactly Joel Embiid, posts up Thomas for a bucket. C.J. Miles.

Part of it is that if most players wind up in a mismatch, it's not a guaranteed bucket. But the Celtics have essentially no room for error. Here, Thomas accidentally gets matched up on Carmelo Anthony on a crucial late possession. Guess how that ended:

And if you bring help, the opponent can reverse the ball, scramble your defense, and then go back to the mismatch:

And then there's a trend that stretches across to the half-court plays. Here Horford knows he has to help down on IT, which leaves the corner for a knock-down shooter:

Watching out for Thomas is a trend. Guys tend to focus a lot on making sure they can help when players isolate Thomas. Here Marcus Smart comes all the way over, giving up an open 3-pointer:

Bradley has missed recent time with injury, but even when he's on the floor, a trend emerges. The Celtics hide Thomas on the lowest threat, and often this means Crowder is chasing perimeter weapons around screens, which has been tough:

Crowder is an incredible on-ball defender, but when chasing players around screens off-ball, he's in the 14th percentile, via Synergy Sports, giving up a 1.25 points per possession mark.

There are a bunch of other factors in here. Jordan Mickey, who will not be in the playoff rotation, has really struggled. Marcus Smart has taken bad angles

It's not one thing. There's not one fix.

So what is the answer, then? Does this mean that Thomas is just too bad to really rely on and the Celtics should bench him or trade him?

Again, absolutely not. Don't be daft. He's Isaiah freaking Thomas, and he's been incredible. This isn't about "Isaiah Thomas is bad." That's a dumb idea. You can win with Isaiah Thomas, and it's important to note in the big picture that they have. The Celtics are 30-18. You can build something really good with Thomas in the middle of it, but you have to have the right parts around him.

OK, so what are some possible solutions?

1. Keep tightening the bolts. The Celtics' issue seems to be that they essentially play up or down to the opponent. They have a 127 defensive rating (and are outscored) when leading by 6-10 points in the 4th. It's not that the Celtics can't play good defense, because when they are tied, their numbers are terrific. They appear to let their guard down when they're up, and perhaps there isn;t as much subbing offense for defense in these situations.

Also, Boston's defense isn't improving. January has seen both their worst overall and 4th quarter defensive numbers of any month this year. Notice that this has also been the month when Isaiah Thomas has gone on this insane tear and established himself as a big-name player, which creates this strange predicament of a player perhaps being the only possible cure for the very problem he helps create.

Boston does have good defenders. Maybe trying a new lineup, one that doesn't use that three-guard combination, will get the job done. Boston's offense was terrible until the All-Star break last year, then they turned it on. Maybe the defense will have the same progression this season.

2. Trade for a big. This is the Pandora's Box for Boston. They want the superstar trade, of that there can be no doubt. But as I wrote a few weeks ago, the timing and availability is difficult. They have so many assets, and they won't all be as valuable later as they are now. (Picks become less valuable when they're actual players after being selected, and simultaneously harder for teams to give up on after the team has committed to them with the pick.) But they also have assets to spare.

Getting a shot-blocker is the biggest thing here. Having a better rim protector to help behind Horford would compensate in those switches and provide Thomas with some cover. He can "die" on screens if there's a beast down low roaming to protect the rim. Some options include Hassan Whiteside, Andrew Bogut, Jusuf Nurkic, Serge Ibaka and Omer Asik. Ibaka and Whiteside would be preferable because you need the mobility, but if they can get a rim protector while only giving up one asset, or moving off expiring contracts, that could tip the scales where they need it.

3. Do nothing. The Celtics are undefeated this season when leading going into the 4th quarter, and are 30-18 overall. They're winning anyway. There's a philosophy that as long as the Celtics are winning games, it's fine. They have the 2nd best record in the East, and a win over the Raptors Wednesday would give them a full two-game lead for 2nd. Does any of this matter?

Yes. It does. This isn't about whether the Celtics can beat the Pistons or Kings or Dallas in January. It's whether their defense will be good enough to hold off teams in the playoffs. Boston's 8-12 record vs. teams .500 or better is telling, as is their 1-8 record vs. the Warriors, Spurs, Rockets, Cavs, and Raptors. Boston can hold off teams with their offense in the regular season. They have to find a better formula for the postseason.

Much of this comes down to what kinds of expectations you have for Boston. When they signed Al Horford, there was talk that they could challenge the Cavaliers. If the Celtics are just going to ride this wave for as long as it will go while focusing on their two draft choices from the Nets this year and next, then they can afford to just let the chips fall where they may. But if they really believe in this team, if they truly believe Thomas is the superstar he certainly looks like, then they have to find ways to put a better winning formula around him.

You can cover for a player's defensive liabilities. The Warriors "hide" Steph Curry regularly, the Rockets do the same with James Harden, the Suns did the same with Steve Nash. But in the Celtics' case so far this season, not only have they been unable to hide Thomas, but doing so has severely hindered the rest of their defense. Most of that is covered up by the incredible highlights Thomas puts up every night, but if Boston wants to make a legit run in the spring, they're going to have to find a way to turn the incredible individual numbers Thomas puts up in the 4th into a wider margin of victory.

Otherwise, we may be talking about how they couldn't hide these problems forever when the postseason comes calling.

Stats provided by and Synergy Sports with additional resources from Basketball Reference.