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

During Wednesday’s game between the Tampa Bay Rays and Milwaukee Brewers, umpire CB Bucknor received a foul ball to the facemask and had to be helped off the field. It was the culmination of a particularly bad week for one of baseball’s most controversial umpires.

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

During Saturday’s game between the Red Sox and Reds, Eugenio Suarez challenged Bucknor on three straight calls and had them successfully knocked down by the ump robot.

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

This was far from the only time Bucknor had his calls overturned at the plate, it was a bad night for him overall. There were eight ABS challenges during the game, and six of them were successful. The two calls that weren’t overturned were extremely close, less than 0.1 inch from the edge of the strike zone. His failures, however, were more dramatic. He called three throws mishits of 2.4 inches or more — one of them was 2.7 inches from the zone. And, in Jomboy Media’s judgment, Bucknor missed 20 calls if you count those that weren’t challenged.

As the game progressed, you could see the exasperation on Bucknor’s face growing as the final five challenges were completed. The clear message from the robots was: you are bad at your job. The league’s reversal rate for ABS challenges is 55 percent, but Bucknor’s rate stands at 78 percent as of April 2.

There have been a few referees whose first encounters with the ABS have taken a turn for the worse. Chad Whitson had all seven of his challenges overturned in his Yankees-Giants game on March 28. But while Whitson may have had a bad game, Bucknor had a rough career.

By UmpScorecards’ metrics, Bucknor has been by far the least accurate umpire in Major League Baseball over the past five years. The site gives each ump an expected accuracy score, then measures the number of correct calls above or below the number they make. From 2020 to April 2, 2026, CB Bucknor is rated 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 almost equally derided Laz Diaz, who is rated 202.03 below the expected number of correct calls over an equal number of matches.

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

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