Much has been made over Aaron Crow's possession or lack of a third pitch that he can use against lefties and how the quality of this third pitch might affect his chances of a successful transition to the rotation.
Crow has a hard sinking fastball and an excellent slider, and against right-handed hitters, he throws these two pitches almost exclusively. And to say it has worked would be an understatement--this combination makes him more or less Death itself against right-handed hitters.
Right-handed hitters have struck out out 27.9% of the time against Crow, and when they do manage to make contact, they've feably chopped the ball into the dirt an unreal 65.1% of the time. As those who have read Fangraph's series on SIERA will know, when a pitcher gets high strikeout rates or high groundball rates, it means that batters are struggling to square up balls, and high rates in either category will correlate with low BABIPS. Crow has been dominant in both K% and GB% against righties, so as we might expect, right-handed hitters are batting only .206 on balls in play with a nearly comical 9.5% line drive rate.
Right-handed hitters have been TPJ on a cold stretch against Aaron Crow. Crow's relationship with batters that stand on the left side of the plate, however, has been another story.
Crow hasn't exactly been bad against lefties, but it's clear that he's allowed lefties to feel much more comfortable. Crow has struck out 19.1% of the left-handed hitters he's faced, and while Luke Hochevar is green with envy, this constitues a 40% reduction in strikeouts when compared to his dominance against right-handed hittrs.
Lefties have also had much more success squaring up balls--they've put the ball on the ground only 46% of the time which is near the league average, and they've hit line drives 23.8% of the time which is about 250% as often as right-handers have been able to make solid contact. Crow has so far mitigated the damage against lefties by walking far fewer, but I'm suspicious as to whether that is a repeatable skill.
Crow's fastball-slider package isn't as effective against lefties as it is against righties, and while this isn't a big problem as long as Crow is in the bullpen, it presents a serious challenge to an attempted transition to the starting rotation. He needs a third pitch to start, and for Crow, it looks like that third pitch will be a curveball.
Recently, Royals officials have become increasingly vocal about trying Crow in the rotation. I figured that this outward bullishness on Crow as a starter should be tied to an increased emphasis on his curve against lefties, so I decided to see if Crow's approach against lefties has changed in the last month plus.
Answer: Crow has thrown far more breaking balls (both curves and sliders) against lefties as the season has progressed, and those breaking pitches have been far more effective.
From the beginning of the season through June 10, Crow threw curveballs 14.1% of the time to left-handed hitters, and only 3.7% of those curves missed bats. He threw his slider 18.8% of the time to lefties and did manage to generate whiffs 19.4% of the time with the slider.
From June 11 through present, however, Crow has mixed his pitches much more against lefties, throwing his curve 20.5% of the time and his slider 27.7% of the time. Meanwhile, his fastball usage has dropped precipitously from 66% to 52%.
Crow seems to have figured out a mix that works--from Opening Day through June 10, lefties swung and missed 8.9% of the time against Crow, but from June 11 to present, they've swung and missed 17.1% of the time. He's essentially doubled his swinging strike rate against lefties in the last month and a half.
A cautionary note--these are very small samples. There were only about 60 sliders/curves in each of the groups I looked at, so I wouldn't count on the whiff rates staying the same. Pitch selection isn't random, however, so even in these relatively small samples, I do think Crow has deliberately shifted his approach toward more curves and sliders against lefties as the team has grown more serious about moving him to the rotation.
And while I doubt Crow's new mix of pitches is twice as effective as the old approach (as the swing-and-miss rate would have you to believe), I do think that it's noteworthy that Crow currently throws a 3-pitch mix against lefties with 50% fastballs, 30% sliders, and 20% curves, and that both his curve and slider have been effective pitches for him.