The ABS Challenge System Reveals Baseball’s Most Controversial Umpire

During Wednesday’s matchup between the Tampa Bay Rays and the Milwaukee Brewers, umpire CB Bucknor took a foul ball to the mask and needed assistance to leave the field. This incident capped off a particularly rough week for one of baseball’s most contentious umpires.

The drama began with the introduction of MLB’s Automated Ball-Strike (ABS) Challenge System, which allows players to challenge balls and strikes for the first time. Each team starts with two challenges per game, and they only lose a challenge if it’s unsuccessful, encouraging players to use them wisely.

In Saturday’s game between the Red Sox and the Reds, Eugenio Suarez challenged Bucknor on consecutive strike three calls and successfully had both overturned by the robot umpire.

Despite Suarez ultimately grounding out, the highlight of the game came from the crowd’s cheers for the successful ABS challenges, overshadowing the Reds’ two home runs. Bucknor had a particularly tough night, with eight ABS challenges resulting in six successful overturns. The two calls that stood were extremely close, within 0.1 inch of the strike zone edge. However, Bucknor’s missed calls were more glaring, with three pitches he called strikes missing by 2.4 inches or more, including one that was a full 2.7 inches outside the zone. According to analysis, Bucknor misjudged 20 calls, even counting those that weren’t challenged.

As the game went on, Bucknor’s frustration was evident as the last five challenges were successful, sending a clear message: he was struggling in his role. While the league average for ABS challenges is a 55 percent overturn rate, Bucknor’s rate was a staggering 78 percent as of April 2nd.

Other umpires have had rough experiences with ABS as well. Chad Whitson had all seven challenges in his game between the Yankees and the Giants on March 28th overturned. However, while Whitson faced a tough game, Bucknor’s struggles have been a long-term issue.

Data from UmpScorecards shows that Bucknor has been the least accurate umpire in Major League Baseball over the past five years by a significant margin. The site tracks expected accuracy scores and measures how many correct calls umpires make above or below that expectation. From 2020 through April 2nd, 2026, Bucknor is rated as being 253.74 calls below his expected accuracy, which is already low. His closest competitor in terms of poor performance is Laz Diaz, who is rated at 202.03 calls below expectation over the same period.

Bucknor’s struggles don’t require ABS to highlight his shortcomings. On Tuesday, he made what many consider the worst call of the young 2026 season, declaring Milwaukee’s Jake Bauers out for not touching first base on a ground ball. This call was quickly overturned on review, but it was clear to everyone watching that Bucknor was mistaken. The only way it could have been more obvious is if Bauers had jumped up and down on the bag. The call was so outrageous that both managers shared a knowing laugh from across the field.

Teams will need to adapt to the new challenge system and develop strategies to navigate it effectively. However, the biggest impact will be on umpires, who will have to adjust to a more consistent and mathematical strike zone. Umpires like CB Bucknor, who have had a more subjective interpretation of the strike zone, are likely to face significant challenges in 2026.