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Inside Aaron Bummer’s Transformation, or How to Make a Lights-Out Reliever From Scratch

A look at the relief pitcher ERA leaderboard yields names many baseball fans are now familiar with. Felipe Vazquez, Adam Ottavino, and Ken Giles have been in the spotlight before. The breakout seasons for All-Star trio Kirby Yates, Shane Greene, and Liam Hendriks have been well documented. Will Harris has been quietly dominant for multiple years. Even Scott Oberg had 200 innings of respectable relief pitching under his belt entering this year.

And then there is Aaron Bummer. Like most of his teammates on the White Sox who don’t routinely flip their bats into near-earth orbit, Bummer is fairly anonymous to the baseball world at large. Yet he has been without question the best arm in a surprisingly respectable White Sox bullpen, notching 49 innings across 41 appearances in 2019, posting a 1.65 ERA that is good for third amongst qualified relief pitchers.

Bummer is an outlier on this leaderboard in more than name recognition. The names ahead of him are racking up the strikeouts, with Yates posting a 14.58 K/9 to go with his 1.08 ERA while Hendriks holds 12.42 and 1.30 marks, respectively. Bummer averages a measly 7.88 K/9 by comparison. Few other relievers with such stellar ERAs have struck out batters at the low rate that Bummer has.

Bummer has largely accomplished this through an astronomical 68% ground-ball rate, surpassed by only worm-burner sensei Zack Britton. So how did this 19th-round relief-only prospect who topped out at 23rd on any White Sox prospect list turn into a ground-ball machine with one of baseball’s most effective fastballs? Let’s take a look. Read the rest of this entry »