Significant trend: Keegan Thompson's changeup usage just spiked as a starter

Keegan Thompson, in his May 17 start against the Pirates, showcased a changeup that could increase his chances to remain a starting pitcher.

As a reliever this season, Thompson has stymied batters by using a four-seam, cutter, or curveball. But as a starting pitcher, Thompson will likely need to develop another secondary pitch to get through the opposing batting order multiple times.

For example, pitchers who use two pitches in at least 70 percent of their repertoire had a wOBA penalty of 0.010 when facing batters for a third time. Keegan Thompson’s four-seam and cutter make up over 70 percent of his repertoire.

When pitchers add a new pitch midseason, their average wOBA reduction is 0.002 against the order for the third time. This doesn’t appear significant, but these analyses are heavily influenced by variation that makes modeling messy. Intuitively, it makes sense that more pitches mean increased difficulty for hitters.

There have been times this season when Thompson has been compared to Justin Steele. Whereas Thompson has one of the best ERAs in MLB, Steele has been inconsistent out of the rotation. Yet unlike Thompson, Steele doesn’t dominantly throw two pitch types. Steele throws a sinker, four-seam, and slider with nearly equal frequency. For this reason alone, the probability Steele can become a five-inning and beyond pitcher appears higher.

But Thompson threw a changeup in 14 percent of his pitches in his most recent start while throwing four total pitch types. This is extremely significant because never before has Thompson thrown so many changeups in one start, as illustrated in the figure below.

As you can see in the video below, Thompson’s changeup has a high amount of horizontal break (8 percent more than league average) without significant dropping action. In fact, the depth on his changeup is nearly identical to the depth of his cutter — the two pitches are essentially mirrors of one another. The difference in Keegan’s changeup and cutter depth is way more similar than the MLB average.

This could be really damaging to left-handed batters. Not surprisingly, Keegan’s first strikeout in his recent start was to a left-handed batter!

Of course, we need to see Thompson’s changeup usage for more than just one start. Claiming that he can suddenly become a valuable starting pitcher based on just a few handfuls of changeups is premature. But the rapid switch in changeup frequency deserves our attention.

Next time you watch Thompson start, monitor his changeup because this pitch could influence his ultimate role as a Cub.