Brandon Nimmo, the Mets' top outfield prospect, was diagnosed with a sprained ACL in his left knee and won't resume baseball activities until he's pain free.

Nimmo sustained the injury Friday while beating out an infield hit in the ninth inning of Double-A Binghamton's game. He awkwardly stepped on first base and, after initially getting up, fell down as he attempted to exit the game, according to the Binghamton Press & Sun-Bulletin. He was helped by teammates down the dugout steps and, on Saturday, sported a knee brace and walked with a crutch, according to the local Binghamton paper.

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Nimmo, 22, was examined at the Hospital for Special Surgery in Manhattan on Sunday.

The Mets' first-round selection in the 2011 draft, Nimmo was hitting .297 with a .368 on-base percentage in 34 games with Binghamton this season. In five minor league seasons, he's batted .271 with a .385 OBP and 22 home runs.

Baseball America rated Nimmo the No. 45 prospect in all of baseball prior to the 2015 season.