DeGrom felt discomfort Friday while throwing a bullpen session at Pittsburgh, manager Luis Rojas said Saturday. An MRI hours later showed there was forearm tightness but no structural damage.
Rojas said Sunday that deGrom will not throw again until the discomfort goes away.
"I'm frustrated," deGrom said. "I don't know what else to say. I guess it's good news whenever structurally, everything looks good, but ... the level of frustration now is really high."
DeGrom is 7-2 with a major-league-leading 1.08 ERA in 15 starts this season despite being hampered by multiple ailments. He last pitched July 7 against Milwaukee.
Rojas added that deGrom said Saturday that he felt similar forearm tightness during a bullpen session right before the All-Star break as well. DeGrom did not attend Tuesday night's All-Star Game at Coors Field in Denver, electing instead to spend time with his family and rest for the second half.
The two-time National League Cy Young Award winner skipped an outing between April 28 and May 9 because of discomfort in his right lat muscle, then did not pitch again until May 25 while spending time on the injured list with what the Mets said was tightness in his right side. DeGrom left his June 11 outing against San Diego after six innings because of flexor tendinitis in his right arm.
Rojas said Saturday that those ailments, including the right forearm tightness, are all unrelated. DeGrom on Sunday said all of his previous ailments this season were due to him swinging, while the new forearm tightness is not. DeGrom is batting .364 in 33 at-bats this season.
Information from The Associated Press was used in this report.