According to acting New York general manager Zack Scott, deGrom threw a bullpen session Thursday and then was diagnosed with the inflammation.
DeGrom reached the high 90s near the end of his session, per Mets manager Luis Rojas, but then didn't throw Friday, a deviation from his routine. He will be shut down from throwing for two weeks, pushing back the timetable of his return to September.
The ace right-hander is 7-2 in 15 starts with a 1.08 ERA, but last pitched July 7 when he struck out 10 in seven innings against the Milwaukee Brewers.
DeGrom has suffered five injuries this season -- shoulder soreness, right lat tightness, right side tightness and right flexor tendinitis were the other ailments -- had two starts pushed back and had an earlier stint on the injured list in May. When he landed on the IL with forearm tightness, he said the previous four injuries had each been caused while batting.
"When I go to release a baseball, I'm having a hard time staying through it and throwing the ball how I'm supposed to," deGrom said in mid-July when the Mets officially placed him on the IL. "The other day, I felt it from literally lobbing a baseball and then never really seemed to get any better. Just continued to stay tight even when I got on the mound. I guess the positive thing is, structurally, my elbow looks good but the frustration part is why. What is it? What did I do to cause it?"
The Mets used their 17th different starting pitcher on Friday night when Carlos Carrasco made his debut for the team. The Mets also wore black uniform tops, a fan favorite, for the first time in nine years.
Carrasco, part of the Francisco Lindor trade in the offseason, missed the first four months with a hamstring tear.
The Mets acquired Rich Hill in a trade last week with the Rays and added Javier Baez and Trevor Williams in a major deadline deal with the Cubs earlier Friday. Williams is 4-2 with a 5.06 ERA in 13 games (12 starts) with the Cubs, and has been hit hard, allowing 10 home runs and a .833 OPS in 58⅔ innings.
Without deGrom, the rotation lines up as Marcus Stroman, Carrasco, Taijuan Walker, Hill and Tylor Megill, with Williams likely heading to the bullpen for now. Megill has pitched well with a 2.04 ERA in seven starts while Walker has struggled of late, with 17 runs allowed in 9⅓ innings over his past three starts.