Keegan Thompson's new slider could be a career-changing pitch

The new slider that Keegan Thompson has added to his repertoire could change the trajectory of his career.

He first threw the new breaking pitch three starts ago, and his confidence in the pitch continues to grow.

“We’ve thrown (the slider) a little more the last couple outings,” Thompson told reporters at Wrigley Field following the Cubs’ 5-3 loss to the Reds on Tuesday. “I got a couple of swings and misses with it (on Tuesday), but I got to get it in the strike zone a little more. Keep working on it.”

According to Cameron Grove’s version of “Stuff+,” Thompson’s new slider rates as a 55/80 (about half a standard deviation above league average). Now that he has added a slider to his arsenal, four of his pitches grade better than league average by stuff.

Thompson’s new slider is significant due to its potential for whiffs. Of all pitch types in MLB, sliders generate the most swings and misses. The swinging-strike rate for cutters and curves — the ones Thompson throws for nearly half os his pitches — is 12.2%, whereas the rate for sliders is much higher at 16.3%.

Thompson’s strike rate (20.5%) is below league average (23.4%).

If Thompson were to throw a slider once every seven pitches, his overall swinging-strike rate would be better than league average, according to predictive models. These models assume his whiff rate would have to be league average, which seems fair because of its 55/80 “stuff” score.

David Ross believes Thompson’s slider success is related to his four-seam fastball.

“Some really bad swings on (the sliders on Tuesday). I thought, for me, his fastball has set up everything,” Ross said. “The fastball, he throws so many early on that when those guys see fastball out of hand, that same direction … It’s the same arm action. He can rip it just like his fastball.”

Ross also mentioned a specific slider — “Somebody swung at one that looked like it didn’t even get to plate,” he said — that was actually highlighted by Rob Friedman (@PitchingNinja on Twitter). As Friedman tweeted, it was a “vicious” one that the batter swung at before it reached home plate.

Like Ross said, the data shows that Thompson’s slider rates high because it plays well off his fastball. “Stuff+” algorithms heavily weigh the difference in horizontal break between sliders and fastballs. Thompson’s horizontal break difference from his fastball is well above league average, as shown below.

Notably, too, Thompson is throwing more fastballs atop the zone since entering the starting rotation a few weeks ago, which could help his slider efficiency going forward.

Thompson was asked postgame when he first started to develop this new slider, and surprisingly, he has only been throwing it for a few weeks.

“We’d mess with it a little bit here and there in the last couple of weeks,” Thompson said. “Been throwing it more in sides, probably the last three or four sides. So about three or four weeks.”

Ross seemed extremely impressed with just how fast Thompson picked up the slider.

“I think the stuff that’s really impressive with guys, showing new grips, new pitches and being able to take those into competition and having the confidence to do that is, I think, unique,” said Ross. “Not everybody has that.”

Thompson certainly has “that.”

After just a few weeks in the Cubs’ “pitch lab,” Thompson might’ve just found himself a potentially career-altering pitch.

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