"Pitcher Noah Syndergaard underwent an MRI this morning at the Hospital for Special Surgery. The MRI revealed a partial tear of the right lat muscle," the Mets said in a statement. "At this point, there is no timeframe for his return. He will be placed on the 10-day disabled list."
Mets general manager Sandy Alderson confirmed Monday that the Mets have placed Syndergaard on the 10-day DL.
"This won't be measured in days, it will be measured in weeks," he said. "It will be a considerable time."
Syndergaard left his start against the Washington Nationals in the bottom of the second inning Sunday, grabbing his right side after throwing a pitch to Bryce Harper. He had allowed five runs on five hits in the first inning during a 23-5 Nationals win.
He had missed his previous start Thursday due to biceps and shoulder discomfort. He had been scheduled for an MRI on Friday but declined to have one after throwing a bullpen session.
The Mets, who open a four-game series at Atlanta on Monday night, did not immediately announce a corresponding roster move after placing Syndergaard on the DL.
Alderson said Sunday that he spoke to Syndergaard after the game but could not say whether the pitcher regretted refusing the earlier MRI.
"We didn't get into that," Alderson said. "I didn't think that was necessary at that particular time, so I think he understands now that there's something going on that needs to be examined.''
Nationals manager Dusty Baker, whose team set a franchise record for runs, said he noticed something about Syndergaard in the first inning.
"We could tell something was wrong," Baker said. "He was throwing the ball firm, but it wasn't moving like usual. When you have a guy like that, you certainly have to jump on him."
Information from The Associated Press contributed to this report.