Perhaps the most important series of the 2018 season will take place this weekend at Fenway Park. Certainly the most important series of the season to date. The first-place Red Sox are hosting the second-place Yankees for four games from Thursday through Sunday.

Here are the top of the AL East standings -- and MLB standings, for that matter -- entering Thursday:

  1. Red Sox: 75-34
  2. Yankees: 68-38 (5 1/2 GB)

The Red Sox and Yankees have the two best records in baseball. This weekend is a chance for the Yankees to gain ground in the division and a chance for the Red Sox to bury New York and create some distance. Both sides would settle for a split. The Yankees, clearly, need the wins more.

Both the Yankees and Red Sox were very active in the days and weeks leading up to the July 31 trade deadline and, in many ways, their moves were geared toward matching up with each other more than anything else. Including this weekend, these two clubs have 10 head-to-head games remaining this season, including the final three games of the year in Boston. The AL East race is far from over.

Here's how the Yankees and Red Sox prepared for this weekend's series and the rest of the regular season at the non-waiver trade deadline.

The Yankees added lefties

MLB: Kansas City Royals at New York Yankees
J.A. Happ gives the Yankees another lefty to throw at the Red Sox. USATSI

Oddly enough, the Red Sox have not had much success against left-handed pitchers this season. They've crushed righties (.271/.338/.469), though their numbers against southpaws aren't as good as you may think:

  • Batting average: .258 (11th in MLB)
  • On-base percentage: .324 (13th)
  • Slugging percentage: .411 (15th)
  • OPS+: 103 (15th)

Kinda of weird for a team with two truly elite right-handed bats (Mookie Betts and J.D. Martinez) and a home ballpark that favors right-handed hitters to have such mediocre numbers against lefties. The Red Sox haven't been terrible against left-handed pitchers. They've been middle of the pack. And, in this case, that means not nearly as good as they've been against righties.

The Yankees imported two left-handed pitchers at the trade deadline: J.A. Happ and Zach Britton. One starter, one reliever. Happ faced the Red Sox twice this season while with the Blue Jays and he's held them to one earned run and a .214/.250/.310 batting line in 10 2/3 innings. Think that caught the Yankees' eye? Sure it did.

I'm not a huge believer in team spits (this pitcher owns that team, etc.) though we now have over 1,000 plate appearances worth of evidence telling us the 2018 Red Sox hit righties much better than lefties. So, at the deadline, the Yankees brought in a left-handed starter as well as another lefty reliever, because that's the best way to beat the Red Sox.

The Red Sox added a righty

MLB: Minnesota Twins at Boston Red Sox
New York's right-handed bats will have their hands full with Nathan Eovaldi. USATSI

In terms of platoon splits, the Yankees have a more balanced offense than the Red Sox. The Yankees lead baseball with a 131 OPS+ against lefties and are second (to the Red Sox) with a 112 OPS+ against righties. Boston's rotation is decidedly left-handed. Their leaders in innings pitched this season:

  1. Chris Sale: 141 innings
  2. Rick Porcello: 131 2/3 innings
  3. David Price: 122 1/3 innings
  4. Eduardo Rodriguez: 104 2/3 innings
  5. Brian Johnson: 60 innings

Porcello and four lefties. Furthermore, lefty Drew Pomeranz returned from the disabled list not too long ago and replaced the injured Rodriguez, so that's another lefty. The Yankees are 5-4 against the Red Sox this season, including 4-2 in their last six games, due in part to how well they match up with all those lefty starters.

So, when the time came to add pitching depth at the deadline, the Red Sox brought in a right-hander who's pitched very well against righties this year: Nathan Eovaldi. Eovaldi, a former Yankee, has held right-handed batters to a .194/.203/.387 batting line with 32 strikeouts and only two walks in 34 1/3 innings this year thanks largely to a relatively new cutter emphasized by the Rays. Aaron Judge, Giancarlo Stanton, Gary Sanchez, Gleyber Torres, and Miguel Andujar aren't looking forward to facing him.

It is entirely possible that, come postseason time, Eovaldi may not be in Boston's rotation. Sale, Price, and Porcello will be in the postseason rotation for sure. That fourth spot is a little up in the air. If Eovaldi is not in the postseason rotation, he could become a lethal multi-inning relief weapon a la Lance McCullers Jr. last year. Either way, the Red Sox needed another righty to counteract New York's lineup, which crushes lefties, and they got that in Eovaldi.

The Red Sox brought in a lefty masher

MLB: Texas Rangers at Boston Red Sox
Steve Pearce has helped the Red Sox get on track against lefty pitching. USATSI

The BoSox made an effort to improve their weakness against left-handed pitching weeks ago, when they acquired utility man Steve Pearce from the Blue Jays on June 28. Pearce's platoon splits:

2018 vs. RHP: .258/.338/.403
2018 vs. LHP: .354/.417/.615

2016-18 vs. RHP: .270/.342/.432
2016-18 vs. LHP: .286/.368/.551

Pearce can hold his own against righties, but he really makes his money against lefties. He crushes southpaws. Crushes 'em. That's why the Red Sox brought him in, to help correct their issues against left-handed pitching. Furthermore, here are some of Pearce's head-to-head numbers:

  • vs. CC Sabathia: .306/.359/.556 with two home runs in 39 plate appearances
  • vs. J.A. Happ: .357/.441/.964 with five home runs in 34 plate appearances

Pearce has faced Sabathia and Happ more than any other pitchers in his career and he has had a ton of success against them. The Red Sox didn't know the Yankees would trade for Happ -- Pearce and Happ were teammates in Toronto, remember -- but things worked out that way. Point is, the Red Sox recognized their vulnerability against southpaws a while ago, and added Pearce to address it.

The Yankees added length to their pitching staff

MLB: Kansas City Royals at New York Yankees
Zach Britton gives the Yankees another dominant late-inning reliever. USATSI

For all the talk about the concerns with their pitching staff, the Yankees came into Wednesday with a 3.56 ERA (4th best in MLB) and 3.68 FIP (4th best in MLB). They're averaging 3.88 runs allowed per game, the fifth lowest rate in baseball. The Yankees are quite good at keeping runs off the board.

That said, the Yankees do have concerns with their pitching staff. They admitted as much with their trade deadline activity, and they attempted to address it by adding two starters in Happ and Lance Lynn, plus another late-inning reliever in Britton. The roster shuffle shakes out like this:

German and Cessa averaged 5.08 innings per start in their 16 starts. Six times in those 16 starts they failed to complete five innings. They wore out the bullpen on the days they pitched, and while the Yankees do have a great bullpen, you don't want to ask it to get 12-plus outs every fifth day if at all possible. Happ can give them more length.

As for Lynn, he had been working as a starter with the Twins, and the Yankees plan to use him as a swingman who occasionally makes spot starts to give the rest of their rotation extra rest. Warren was an effective middle innings guy but he couldn't start. Lynn is better able to give the Yankees innings. Britton is flat out better than Shreve -- considerably better -- so those are more high quality innings.

The Yankees were hoping to land an ace at the deadline, or at least someone close to an ace. A Jacob deGrom type. That pitcher was not available, though -- I suppose he could've been depending on your opinion of Chris Archer, but I digress -- so the Yankees did the next best thing. They upgraded the three weakest spots on the pitching staff. The soft underbelly isn't so soft anymore.

Still more moves to come?

It is very possible. Trades can still be made in August (here's how those work) and both the Yankees and Red Sox still have some needs. The Yankees could use another outfielder and a catcher to help cover for the injured Judge and Sanchez, and the Red Sox could still use another bullpen arm. Those are things that can be addressed with an August waiver trade, and given how competitive the AL East is, you can bet both teams will continue to look for any upgrade possible.