The Celtics' rotten injury luck started five minutes into the season, when Gordon Hayward went down with that gruesome broken ankle. Now it's picking back up at the worst time. With the playoffs right around the corner, Kyrie Irving is out for now with knee soreness, Jaylen Brown is out indefinitely with a concussion after taking that nasty fall on his head and upper back, Daniel Theis is reportedly done for the season with a torn meniscus, and now come perhaps the worst news of all. 

Marcus Smart may be done for the year. 

This isn't a sure thing. The Celtics reportedly just don't know. They're waiting for a second opinion on the torn tendon in Smart's right thumb. But as far as probabilities go, this doesn't sound great:

Now, before we go off the deep end, let's keep things in perspective. Irving is day to day, and the Celtics are confident it's just a tendinitis issue with his knee, so he should be fine for the playoffs. Brown will presumably be back by mid-April as well. Theis has been a surprisingly key big man off the bench, and he'll be missed for sure, but Greg Monroe should be able to reasonably replace at least some of what he brought. And besides that, losing Theis is not going to be the difference between competing for a championship and not. 

Smart is a different story. 

Smart has long been a basketball enigma, a terrible shooter who for significant stretches of the season has made the Celtics a better offensive team when he's on the floor, a terrific defender who at times doesn't actually impact the defensive numbers all that much (though entering Tuesday Boston is now four points stingier on defense when Smart is on the floor). 

The bottom line is this: Smart is a basketball player, the Celtics are a team full of basketball players, and in this way, he is the Celtics. The proverbial heart and soul. The immeasurable, and invaluable, commodity. Losing him would very likely be too much to overcome in terms of the Celtics competing for a title, which they're already a long shot to do in the first place. 

When it gets down to it in the conference finals, or the Finals if they were to make it that far, the Celtics are not going to out-talent the elite teams, though their collective talent isn't that far off the elite tier. Irving is an All-NBA player. Brown and Jayson Tatum will both be All-Stars in the near future. Al Horford is a Defensive Player of the Year candidate and one of the best passing bigs in the league. Terry Rozier is one of the best bench guards in the NBA. But Smart is the one who tends to bring it all together. 

He's going to get you the extra possessions. He'll win you the swing defensive possession. And, yes, he'll get you a big bucket when you need it. He's good with the ball in his hands. There are loads of guys who shoot better than him who simply don't impact the offense the same way. It doesn't make sense on paper, but it's real. And the Celtics, if Smart is in fact out for a significant amount of time, will miss him tremendously. 

Put it like this: If Smart were to be ruled out for the year, and the Celtics were to face, say, the Sixers in a first-round matchup, that could very well turn into a coin-slip series. That's the difference Smart could make. A team that could compete for a berth in the Finals would be battling for all its worth to get out of the first round. Heck, they might find themselves in a six- or seven-game war with Philadelphia with Smart. 

As it stands entering Tuesday, the Celtics would match up with Milwaukee in the first round. That is also a very scary matchup for Boston. For anyone, really, given Giannis Antetokounmpo's presence and all their length on the perimeter. That series could very well come down to the wings sort of canceling each other out, the Celtics having an advantage with Kyrie, and the Bucks having a giant advantage with Giannis. 

Smart could be the swing player in a series like that. 

Really, that's what he is: A swing player. The difference between winning and losing. Honestly, if he's just on the court the Celtics will take it. Even if he's not 100 percent, he's a guy that will pretty much grind the same way. The injury is on his shooting hand, but Smart can't shoot anyway. We'd have to see how it affected his ball handling. Anyone who's watched him even a little bit knows he'll be aggressive either way. 

That, in fact, is one of Smart's best qualities. He will take his shot every time it's there, numbers be damned. And that keeps a defense honest. Coach Brad Stevens has said he trusts Smart to make the ones that count, and that keeps defenses honest, too. Smart has that feel of a player that will hurt you if you neglect him. Somehow. Some way. If he's not in there, the only ones hurting are going to be the Celtics.