Leaving Safeco Field T-Mobile Park (good luck getting used to that) to call the Bronx home will be a new challenge for James Paxton. Seattle is not an easy place to hit homers, much unlike Yankee Stadium. This is somewhat worrisome because Paxton had some issues with the long ball last season. Yet, home run prevention was once one of the southpaw’s strengths. He allowed only 0.7 homers per nine innings leading up to 2018, but suddenly permitted 1.3 per nine in 2018. Was it a blip? Hopefully. Maybe last year was just “one of those years”.
Even if he had some misfortune last season, Paxton inevitably will allow more homers as a Yankee than as a Mariner. He’s already gotten a taste of what Yankee Stadium can do to pitchers:
Those short porch specials are going to happen and they be part of the reason he’ll allow homers in pinstripes. It’s just a question of which Paxton will experience the increase: the pre-2018 version, his 2018 self, or something in the middle? If it’s the pre-2018 version, we might only be talking about 14 or 15 homers allowed all year, which is still great. If it’s a jump over what happened in 2018, he could allow 26 or 27, which is a lot.
A fun thing to do is to look at an overlay of the line drives and fly balls Paxton allowed last season on an over Yankee Stadium’s dimensions. This might look scary, given how many of the non-dark red dots are over the wall in the Bronx. Not to fear though, it’d be silly to count those up an assume that’s going to be his increase over last year. After all, he’s not going to make all (hopefully) 30-plus starts at home. It’s an interesting chart to look at, and easy to understand, but we have to look deeper to figure out why the lefty gave up so many more homers in 2018.
Batters hit more fly balls against Paxton than ever in 2018, increasing from 32.7% to 41.1% year-over-year. That on it’s own will lead to more home runs. More fly balls are more opportunities for dingers. Certainly that helps us understand why he went from nine to 23 home runs allowed, but there’s more to it.
It’s not surprising that he gave up more fly balls because Paxton increased the elevation of his pitches last season. What is surprising is the jump in home runs allowed per fly ball (HR/FB rate). Paxton always fared better at homer suppression than the rest of the league up until last season, with the exception of 2013. Let’s just ignore that first season because he only tossed 24 innings.
| Year | Paxton | League |
| 2013 | 12.5% | 10.5% |
| 2014 | 6.4% | 9.5% |
| 2015 | 11.1% | 11.4% |
| 2016 | 8.2% | 12.8% |
| 2017 | 7.8% | 13.7% |
| 2018 | 14.4% | 12.7% |
To this in perspective, if Paxton maintained his 7.8% HR/FB ratio from 2017 in 2018, he’d have allowed a dozen dingers, a small increase from nine in 2017. In an overly simplistic way, that essentially attributes three home runs to the increase in fly ball rate. How do we make up the gap for the other eleven? Obviously, the higher rate of homers per fly ball is the big culprit. That leaves us to figure out why that ratio spiked.
It makes sense to hone in on his fastball and cutter to investigate his home run woes. He throws those pitches about 75 percent of the time and rarely allows homers on anything else. I particularly wanted to see how his HR/FB rate changed based on pitch location, so I pulled all of Paxton’s balls in play over the last two seasons from Baseball Savant. Using Z location, which measures the vertical placement of any pitch, I binned all of Paxton’s batted balls by height. For reference, the typical hitter’s strike zone bottoms out at approximately 1.3 feet from the ground and peaks at 3.3 feet from the ground. All but two of Paxton’s homers allowed on fastballs since 2017 were given up in the 1.75 to 3 range, so I focused there.
After not allowing homers in the 1.75 – 2 and 2.75 – 3 ranges last season, Paxton allowed six. There were more fly balls hit, especially in that top range, so it’s not surprising that a few balls found their way out of the park. Still, it’s apparent Paxton ran into some bad luck. In both regions, his actual wOBA allowed on all balls in play was higher than his xwOBA. At the higher region, it was .457 vs. .360, and the lower region, .385 vs. .329.
Now let’s move toward the heart of the plate. From 2 – 2.5, Paxton allowed ten homers on thirty fly balls compared to just four on twenty the season prior. This somehow occurred despite a lower xwOBA in 2018 (.408) than 2017 (.432). In other words: in a season when hitters had poorer quality of contact when they put the ball in play on pitches in that range, Paxton had worse luck.
How can we interpret this information? For one, maybe Paxton was lucky in 2017. Perhaps he should have given up more home runs and that his low HR/FB rate was good fortune. Or, the lefty was really unlucky in 2018 and he truly is skilled at keeping fly balls in the yard. The true answer lies somewhere in the middle of those extremes. Seattle almost certainly helped his HR/FB rate in the seasons prior to 2018, but that doesn’t mean he deserved the results he had in 2018.
Paxton will probably wind up somewhere in the middle of the two versions of himself while in pinstripes. That’s a cop out answer, I know, but it’s sensible. Yankee Stadium gives up more homers, so that’s an inevitable boost to Paxton’s HR/FB ratio. It’s just that we shouldn’t use 2018 Paxton as a baseline for the increase. Projection systems PECOTA (1.09 HR/9) and Steamer (1.17 HR/9) agree, though they are a little bit closer to 2018 Paxton than pre-2018 Paxton.
If all of this makes you worried about Paxton in pinstripes, take a deep breath. Keep in mind all of the good things that he does. He’s got a blazing fastball, a sharp cutter, and good control. He misses a ton of bats. Projection systems like him too, basically just as much as Luis Severino. As long as he stays healthy, there’s no reason to believe that the southpaw won’t excel in pinstripes. He’s too talented not to.