The Mets announced the move before Saturday's game against the St. Louis Cardinals. All-Star outfielder Michael Conforto was activated from the DL in a corresponding roster move, though he did not play in Saturday's 4-1 loss to the Cardinals
Mets manager Terry Collins told reporters that Nimmo was sent to the hospital Friday night after complaining about how he felt following New York's game in St. Louis.
"[The diagnosis] is not necessarily real serious, but it's surprising for sure," Collins told reporters.
Nimmo, 24, has appeared in 15 games for the Mets this season, batting .350 (7-for-20) with two RBIs. He was the Mets' first-round draft selection (No. 13 overall) in 2011.
Conforto, who had been sidelined since June 25 because of a hand injury, was not in the Mets' starting lineup Saturday against the Cardinals. He is batting a team-best .285 with 14 home runs and 41 RBIs this season.