James Paxton has some peculiar platoon splits

As any good baseball nerd, when the Mariners and Yankees announced the Paxton deal, I looked up statistics for both James Paxton and the prospects involved in the trade who were going to the Mariners. Although I did not end up writing about the trade I looked into it in order to form my own opinion on it. I read multiple articles on analyzing the trade, and dug into the numbers, and something caught my eye.

A fair concern I saw brought up was how Paxton would adjust to pitching in Yankee Stadium, which is a far, far more hitter-friendly ballpark compared to Safeco Field. I saw someone arguing that Paxton being left-handed will mitigate left-handed hitters taking advantage of the short porch in right field. That sounded like a reasonable statement to me, but I decided to check on Paxton’s career splits anyway. I was surprised by what I found.

Paxton has reverse splits for his career. This past season was a major contributor to that, as he had a .377 wOBA vs. left-handed batters and a .268 wOBA vs. right-handed batters.

This is really weird. I have written numerous times about how hitters cannot have true talent reverse splits, according to research done in The Book. Pitchers, however, can have true talent reverse splits, as there is a lot more variability that goes into pitching compared to hitting. Due to variable pitching repertoires, how pitches are thrown, arm angles, and mechanics, pitchers’ true-talent reverse platoons are sustainable.

Projecting the Mariners’ return for James Paxton

Photo by Mike Stobe/Getty Images

Many said that the Mariners received a “light” return. Is that truly the case?

Reverse splits are most common in pitchers who have an excellent changeup, as that pitch is commonly used to get opposite-handed hitters out. Johan Santana is a great example of this. He had a legit 80-grade changeup which might be one of the best ever. For his career, lefties had a .295 wOBA against him, while righties had a .276 wOBA.

The thing with Paxton is that he almost never uses his changeup. According to Brooks Baseball, he has used it only 5.5 percent of the time in his career. Last year he only threw it ten times total.

So how is Paxton doing this?

While the first inclination to answer this might simply be to say it is a small sample, that actually might only be a small part of it. Pitchers’ platoon splits vary a lot more than that of hitters. There is very little variability in hitters’ true talent splits, which is why you need so many plate appearances to conclusively say that a hitter’s platoon skills are significantly different from the mean (2,000 PA vs. LHP for righties, 1,000 PA vs. LHP for lefties).

Since pitchers’ platoon skills can vary so much, you can draw conclusions on a pitcher’s platoon skills based on far smaller samples. Generally speaking, according to The Book, a right-handed pitcher needs 700 plate appearances against lefties, while a left-handed pitcher needs just 450 plate appearances against lefties. Though, to be clear, this is not a hard, fast rule, the greater platoon variability that allows for smaller samples also means that there is more variability around what a significant sample size is.

Paxton has faced a total of 420 left-handed hitters in his career. Because calculating his platoon projections goes beyond my already limited expertise, I can’t provide them for you. What I can say is that it is possible that Paxton possesses true talent reverse splits, but I doubt they are as large as 50 points of wOBA. Again, this is speculative without being able calculate the projections.

When the authors of The Book were analyzing their research on pitcher platoon skills, they made a few observations on those with reverse splits. They tended to not have low arm slots, with half of these reverse split pitchers relied on their fastball, while others relied on a sinker and/or curveball. That’s basically how you would describe James Paxton. He has a fastball that sits 95-96 but can touch 100. He has a great knuckle curveball, too. For his career, he used his fastball 61 percent of the time, and his curveball 18 percent of the time. This past season that changed to 56 percent and 22 percent, respectively. Either way, roughly four out of every five pitches that Paxton throws are the kind that can potentially negate or reverse splits.

It is important to keep in mind that different pitches play off of each other. That being said, let’s see how hitters have performed against Paxton’s fastball and curveball for his career.

Now let’s take a look at just the 2018 season. It is a much smaller sample, obviously, but lefties hit Paxton over 100 points of wOBA better than righties.

Those data are pretty interesting.

So does this mean that teams should stack their lineups full of left-handed hitters whenever Paxton pitches? It is tough to say. Or rather, it is tough for me to say. The research done in The Book was done back before we had access to Pitchf/x data. With access to these data as well as pitchers’ arm slots, teams’ analytics groups can estimate a pitcher’s platoon skill more accurately than doing so simply by handedness. It will be interesting to see how opposing teams set their lineups against Paxton in 2019.