Even Major League Baseball umpires are adjusting to the new rules.

On opening day Thursday, the umpires called a strike against the New York Mets' Jeff McNeil because of a runner on first.

The umps initially ruled Pete Alonso did not get back to first base in time after a pitch. That put McNeil in an 0-2 hole despite only one pitch being thrown.

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Buck Showalter with umpires

Manager Buck Showalter of the New York Mets talks with home plate umpire Paul Emmel bout a call during the seventh inning against the Miami Marlins June 24, 2022, in Miami, Fla. (Eric Espada/Getty Images)

However, manager Buck Showalter said Friday the umps made a mistake, and a strike should not have been called.

Under the new rules, pitchers have 15 seconds to deliver a pitch, 20 with one or more runners on base. Batters have eight seconds to be "alert" in the batter's box.

But the umps admitted to Showalter that a strike should not have been assessed to McNeil, who hit an RBI single anyway.

Pete Alonso rounding bases

Pete Alonso of the New York Mets rounds the bases after hitting a home run against the Miami Marlins during the ninth inning at loanDepot park March 31, 2023, in Miami, Fla. (Megan Briggs/Getty Images)

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Showalter also said umps will start to issue warnings if baserunners are taking their sweet time.

"When I refereed basketball, it’s called preventive officiating, where you go, ‘Hey, get out of the lane,’ instead of blowing the whistle," Showalter said Friday. "If they keep doing it, you pop them." 

Buck with umpire

Manager Buck Showalter of the New York Mets argues a call with umpire Larry Vanover during a game against the Washington Nationals at Nationals Park Aug. 1, 2022, in Washington, D.C.   (Mitchell Layton/Getty Images)

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The Mets won the game, but fell 2-1 Friday to the Miami Marlins.