"He let us know earlier today that he had some tightness on his right side," Mets manager Luis Rojas told reporters about three hours before Tuesday's game, which was later postponed due to inclement weather.
DeGrom underwent an MRI, which revealed inflammation in his right lat. The team said he will refrain from throwing for the next few days.
"I saw him before coming to the field, and he was stretching, trying to feel his right side a little bit, and that's when we started discussing whether he's going to start or not," Rojas said. "And then we got to the point where he's not [starting]."
The 32-year-old deGrom has been one of MLB's most dominant pitchers this season. He has a 2-2 record and league-leading 0.51 ERA over five starts, with 59 strikeouts in 35 innings.
The two-time National League Cy Young Award winner has reached 100 mph on 46 of 473 pitches this year and lowered his career ERA to 2.55, surpassing Baseball Hall of Famer Tom Seaver (2.57) as the Mets' career leader.
Right-handed reliever Miguel Castro was set to start in place of deGrom on Tuesday night against the Cardinals before the postponement. The game will be made up Wednesday as part of a doubleheader.