I Strike Out Opposite-Handed Batters Now. Does That Even Matter To You?

In 2017, Gerrit Cole struck out opposite-handed batters, lefties, at a 22.8% rate. That mark was above the MLB average, 20.4%, among right-handed starters striking out left-handed hitters. In the first few months of the 2018 season, March, April, and May, Cole’s K% versus left-handed hitters rocketed up to 45.5%.

Here’s how he did it:

Beyond forcing lefties to whiff more often on every pitch he offered to them, Cole made a few changes to the way he approached lefties. For one, he threw a much better four-seamer more often. That has little to do with splits and more to do with the development of a great pitch. More specific to his splits, he stopped throwing his changeup and sinker to lefties as often and increased his usage of the knuckle curve. The results? Cole’s 2018 season overall (no splits) ERA, WHIP, and K% all finished better than they had the previous season. Take a look:

• 2017 ERA: 4.26 -> 2018 ERA: 2.88
• 2017 WHIP: 1.25 -> 2018 WHIP: 1.03
• 2017 K%: 23.1%  -> 2018 K%: 34.5%

There’s no way we could conclude that Gerrit Cole’s 2018 success was due to the fact that he generated more whiffs against lefties, but it didn’t hurt. We know the story is more about overall performance as Cole moved on from Pittsburgh and joined a Houston Astros team that was teaching their pitchers how to effectively spin the ball. Gerrit Cole simply became a much better pitcher when he moved to Houston.

But if an early-season improved K% against opposite-handed batters is part of the story of an improved pitcher, that seems worth evaluating. In an article I wrote a few weeks ago, I presented pitchers who displayed improvements in their 2025 split K%’s over 2024’s final mark. You can see that data here. Much in the way I report fastball velocity improvers, I thought I’d find a way to automate the data collection from the split K% improvers and report my findings regularly. Off I went to boast about how identifying early split improvers can help find early breakout pitchers when I was stopped dead in my tracks by a friend who said, “But does it actually matter? Like, in the grand scheme of things…is it predictive of success?”

For this analysis, I’ve collected data from March/April and May 2016-2024 from pitchers with at least 10 IP in those early-season months. I then merged in the previous full season’s data to identify pitchers who improved their ability to get out opposite-handed batters in the first few months of the season, compared to what they had done the prior season. To qualify for this analysis, a pitcher had to have thrown at least 100 IP in the season prior to throwing at least 10 IP in March/April/May. This season’s dataset will be incomplete as May 2025 has only just begun. More on that later. Here are a few rows of data to help visualize the sample:

Again, Gerrit Cole displayed a huge jump in his ability to strike out opposite-handed hitters in the first few months of the season back in 2018, compared to what he did in the full season of 2017. Let’s get the obvious things out of the way first. 10 IP early in the season is a small sample. True. Survivor bias will greatly influence this analysis, as anyone who threw 100 stinker innings in a season likely doesn’t get the chance to throw 10 more early in the following season. We can see that instantly, as the split matchup K%’s on display are all hovering around a typical ~20% league average. In the table above, Mitch Keller stands out. In a full 2022 season, Keller (a righty) didn’t have a great pitch to throw against lefties. In 2023, he developed a cutter and threw it to lefties 24% of the time, earning a 12.7% SwStr%, better than league average.

Now that we understand how I collected the data and have had a chance to glance it over, let’s get to analyzing. The question is: Do early-season signs of improved split K% (LHP vs. RHH / RHP vs. LHH) indicate full-season improvements in overall ERA, WHIP, and K%? The dataset I munged together contains 44o observations, 326 righties throwing to lefties, and 114 lefties throwing to righties. Here’s how the groups break out:

First, we see that it’s not that uncommon for a pitcher to improve their K% early in the season against opposite-handed batters, but that it’s less common than declining. That goes for both right-handed and left-handed pitchers. Furthermore, both the K% improvements and declines are around 4%. Let’s now look at how this early change has an effect on the full season, starting with right-handed pitchers:

This is not what I was expecting. Of those who declined in their split K% early in the season, their ability to strike out opposite-handed hitters, 64.5% of them finished the season with a better ERA than they had in the previous season. The decliners best the improvers in ERA, WHIP, and end-of-season K%. This feels somewhat odd, but when you consider the entire season’s context, you get a sense of how things level out. Let’s look at Charlie Morton as an example.

In 2021, Uncle Charlie struck out lefties at an absurd 32% clip. When he showed up to work in 2022, that mark had dropped to a still respectable 22% through April. By the time June rolled around, Morton was setting lefties down over 40% of the time:

Morton’s end-of-season ERA, WHIP, and overall K% weren’t bothered by the slow split K% start. The full season leveled things out. The same story can be told from the left-handed pitchers. Those with improved split K% metrics early in the season didn’t show significantly more improvement in ERA, WHIP, or overall end-of-season K%.

With more time, I might be able to find connections between early split K% and season-long success beyond this naive approach. But, it seems much more practical to simply conclude, it’s a long season. Split K% fluctuates throughout it, and just because a pitcher comes out of spring training with a new weapon to throw to opposite-handed batters, it doesn’t mean it will be a weapon all season long. Hitters adjust. Pitchers lose their feel and get it back. The world keeps spinning.

Furthermore, the rules of this analysis aren’t tight enough. When I change the parameters and add a 5% or more requirement for defining improved early-season split K% as improvers, the results are the same. Too much tweaking and I’m heading down the self-fulfilling avenue in a hot rod. I’ll leave it where it is for now.

There’s not enough evidence to motivate a continued, regular posting of “Hi, I Strike Out Opposite-Handed Hitters Now” because, well, we probably shouldn’t care. At least not until it becomes a sustained thing. But, if you’re the type of reader who skips to the bottom and simply looks for names to consider claiming from the waiver wire, this part is for you!