The overwhelming underperformance of Aaron Bummer, explained

Traditional statistics suggest the southpaw is having a bad year, but advanced stats tell a different story

Few players embody the pure disappointment of the White Sox’s 2023 season better than South Side southpaw Aaron Bummer.

After four consecutive seasons of top-shelf performance, Bummer’s surface-level stats have taken an unprecedented nose dive. The same reliever who was, on average, 50% more valuable than a standard reliever for the past four years now sports an unsightly 7.11 ERA and has played himself out of usage in high-leverage situations.

Is he really this bad, or is there hope for some regression to the mean in the future?

Despite his 7.11 ERA, Bummer’s Fielding Independent Pitching (FIP) is holding steady at a miniscule 2.31, and his Statcast expected ERA is a tick higher at 3.09. In lay terms, Bummer’s inflated ERA is about four entire runs higher than what his peripherals suggest it should be. Why does this deviation exist, and is it fixable?

To puzzle together the answer to the first question, we should first take a look at opposing hitters’ batting average on balls put in play (BABIP), against Aaron. If he's simply getting unlucky results from random grounders, this statistic will be a good indicator.

League average BABIP typically holds steady around .300. Bummer, on the other hand, is sporting an ugly .372 BABIP against that suggests he’s been incredibly unlucky on balls put in play this season. For a sinkerballer who relies on keeping the ball on the ground, a high BABIP can certainly be a factor in an inflated ERA.

That's not the whole story, however. Bummer has also allowed a career-high 49% hard hit percentage this year that puts him in the bottom five percent of the league in that category, indicating that nearly half of the balls put in play against him are squared up and hit hard despite being on the ground. This puts stress on an unsteady defense, and furthers the chance that a ball put in play on the ground sneaks through for a base hit, or worse.

How are batters squaring him up so frequently? Well, all five of Aaron Bummer’s pitches have graded out at about average in terms of run value this year. The only pitch he throws that has been slightly above-average to this point in 2023 is his cutter, which he only tosses 12% of the time. As the chart below demonstrates, about 92% of the time Bummer is deciding to throw average or below-average pitches that batters square up, put on the ground, and push through. In fact the majority of the time Bummer is going with his sinker, and to his own peril.

The main culprit behind Bummer’s hard-hit problem, it seems, is underwhelming sinker location. As you can see below, Bummer’s sinker (a pitch he throws nearly 60% of the time) is catching a lot of the plate in 2023. A sinker that catches the plate typically finds itself perfectly within a modern hitter's bat path, especially considering the outsized emphasis on upper-cut swings and launch angle today.

A rare silver lining in Bummer’s game this year that at the same time reinforces the hard-grounder theory is the fact he has a 99th percentile barrel rate. (A “barrel” in advanced statistics is a ball hit 98+ mph at a launch angle between 26 to 30 degrees.) Few barrels and a high hard-hit percentage suggests that opposing batters are getting hard contact off of Bummer but not good contact — almost never at an angle that would send it out of the park.

Poor location.
Hard contact.
Directly into the ground.
Through for a knock.

Now that we’ve identified this seemingly anomalous problem, is it fixable? The short answer is a resounding maybe. While a normal pitcher’s BABIP would regress to the mean more often than not, extreme ground ball pitchers like Bummer typically sport a higher BABIP than pitchers who rely on fly balls or strikeouts (runner speed being a factor in beating out balls that are caught in the air, for one thing). However, it is encouraging that Bummer’s career BABIP is a serviceable .304, and any regression towards that mean can only benefit the Pale Hose’s unlucky lefty.

While a degree of positive regression is certainly possible, don’t count on it unless Bummer is able to rectify his control issues and command the zone with more authority and consistency. Regression is less likely to happen if Bummer continues catching the plate and allowing hard contact.

Despite what advanced statistics may suggest, Bummer has earned his career-worst ERA to this point. However, something as simple as a slight tweak in his mechanics that results in better command after the All-Star break could return the once-dominant lefty reliever to his former glory.