Unpacking Aaron Bummer's Challenging 2023 Season

There was once a time when Aaron Bummer looked to have the brightest future of any reliever in the White Sox bullpen. 

How the times have changed. Bummer went from a late-inning weapon to liability in the span of one season. It was one of the many reasons a seemingly stacked bullpen on paper ended up being a weak point of the team.

To say the 2023 season didn’t go the way Bummer had wanted was an understatement. The 30 lefty posted a 6.79 ERA in 61 appearances. In those 58.1 innings of work, he carried a 1.53 WHIP and averaged 5.55 walks per nine innings. 

Bummer’s struggles had nothing to do with his stuff. In fact, his strikeout rate and whiff rate ranked in the MLB’s top 87th percentile. His 12.03 K/9 innings was also the second-highest mark of his career. 

Bummer also carried a respectable expected ERA of 3.53 and an expected batting average of .215 which suggests that opponents struggled to make hard contact against him. This is further reinforced by the opponent's 2.7 barrel percentage against him, placing Bummer in the MLB’s 99th percentile. 

Bummer also did a good job keeping the ball on the ground, which is especially important when pitching in a hitter-friendly park like Guaranteed Rate Field. The former Nebraska Cornhusker had a 601 ground ball percentage which placed him in the league's 97th percentile. 

Unfortunately, the box score does not care about xERA, xBA, or ground ball percentage. The bottom line was that Bummer gave up a lot of runs. 

The ability to avoid barrels doesn’t mean anything if you are avoiding the strike zone as well and Bummer's kryptonite came in the form of walks. The Valencia native carried a 13.5 percent walk rate which was among the worst in the league. It was also his highest walk rate since his rookie year. All those free passes got Bummer into trouble which he was rarely able to work around. 

He did have some bad luck with soft contact resulting in hits. But that would not have been as much of an issue had he not been constantly trying to pitch out of traffic on the basepaths. 

Another reason for his struggles was his sinker. Ethan Katz once called Bummer’s sinker “the most impressive thing I have ever seen”. There was good reason for this. It has 4.3 inches of vertical movement and comes in at 94 mph.  In 2019 Bummer’s sinker carried a Run Value of 20. This season his sinker had a Run Value of -4. That’s a drastic turnaround in the span of just four seasons. Opponents are no longer fooled by it. Unfortunately for Bummer, he relies on his sinker more than any other pitch. 

He threw his sinker 498 times in 2023. Opponents had a .323 batting average against it. His sweeper was Bummer’s most effective pitch. He threw it 418 times and produced a Run-Value of 5 and an opponent average of just .160. 

But Bummer can’t lean solely on his sweeper. He needs a quality secondary pitch to complement it. That pitch used to be his sinker. Now he may have to consider looking elsewhere, especially if he struggles to throw strikes. 

The other three pitches he relied on were his fastball, cutter, and changeup. It was very apparent he did not have much confidence in any of the three. He threw his cutter just 10.5 %, fastball 5.6% and changeup 0.5%. 

Bummer is under contract until 2026 with two club options after the 2024 season. There is plenty of reason to believe that he can return to the late-inning setup man that the front office envisioned when they signed him to a long-term extension. But for that to happen he is going to need to make some changes during the offseason and that starts with throwing more strikes.