MLB’s ABS Challenge System is exposing the worst umpire in baseball

During Wednesday’s game between the Tampa Bay Rays and the Milwaukee Brewers, umpire CB Bucknor took a foul ball to the mask and had to be helped off the field. It was the cap to what has been a particularly bad week for one of the most controversial umpires in baseball.

It started with perhaps the best example of how MLB’s Automated Ball-Strike (ABS) Challenge System can inject drama in baseball. This is the first year for the new robot umpire, which allows hitters, catchers, and pitchers to challenge balls and strikes for the first time. Each team starts a game with two challenges. But they only lose a challenge if it’s unsuccessful, so players aren’t inclined to demand a review unless they’re sure.

During Saturday’s game between the Red Sox and the Reds, Eugenio Suarez challenged Bucknor on back-to–back strike three calls and successfully had them overturned by the robo ump.

It doesn’t matter that Suarez ultimately grounded out. What matters is that, in a game where the Reds hit two home runs, the loudest cheers came for a pair of successful ABS challenges.

This was far from the only time that Bucknor had his calls overturned at the plate, it was a bad night for him overall. There were eight ABS challenges over the course of the game, and six of them were successful. The two calls that were not overturned were extremely close, within 0.1 inch of the strike zone edge. His misses, however, were more dramatic. Three pitches he called strikes missed by 2.4 inches or more — one was a full 2.7 inches out of the zone. And, by Jomboy Media’s judgment, Bucknor blew 20 calls if you count ones that weren’t challenged.

As the game progressed, you could see the exasperation on Bucknor’s face grow as the last five challenges were successful. The clear message from the robots was: you’re bad at your job. The league rate for ABS challenges is 55 percent overturns, but Bucknor’s rate stands at 78 percent as of April 2nd.

There were a couple of umpires whose first brushes with ABS went worse. Chad Whitson had all seven of the seven challenges in his game between the Yankees and the Giants on March 28th overturned. But while Whitson may have had a bad game, Bucknor has had a rough career.

According to measurements by UmpScorecards Bucknor has been the least accurate umpire in Major League Baseball over the last five years by a significant margin. The site gives each ump an expected accuracy score and then measures how many correct calls above or below that they make. From 2020 through April 2nd of 2026, CB Bucknor is rated as being 253.74 below his expected number of correct calls — and his expected accuracy is not particularly high to begin with. Bucknor’s closest competitor is the nearly as derided Laz Diaz, who is rated as being 202.03 below the expected number of correct calls over an equal number of games.

Bucknor doesn’t need ABS to make him look bad, either. On Tuesday, he made what is clearly the worst call of the young 2026 season so far. He called Milwaukee’s Jake Bauers out for failing to touch first base on a groundball. The call was quickly overturned on review, but you didn’t need instant replay to know that Bucknor was wrong. The only way it could have been more obvious is if Bauers stopped and jumped up and down on the bag with two feet. The call was so egregious that both managers exchanged a knowing laugh from across the field.

Teams will obviously have to adjust to the new challenge system and develop strategies for how best to navigate it. But its biggest impact will be on umpires who are going to have to adjust to a more consistent and mathematical strike zone. Umpires like CB Bucknor who have had, let’s say a more subjective view of the strikezone, are going to have a rough 2026.