Showalter was still awaiting word on results of the exam shortly before the start of Friday's spring training game against the St. Louis Cardinals. The manager had said Thursday that deGrom felt tightness in his right shoulder while throwing long toss.
The timing is far from ideal for the Mets, who open the season in a week, with deGrom slated to pitch in the opener against the Washington Nationals. Showalter is already working on contingency plans should the MRI results prove unfavorable.
"Does he need to pitch another live game before he opens the season? In a perfect world, yes, we'd like to see him do that -- and recover from that," Showalter said of deGrom.
If deGrom can't pitch on Opening Day, the ball would almost certainly go to Max Scherzer, who signed with the Mets in November.
Showalter said he wanted to wait until after Scherzer throws a simulated game of seven innings and fewer than 100 pitches against Mets minor leaguers Saturday before committing to that possibility.
The spring rotation, which also includes Chris Bassitt, Carlos Carrasco and Taijuan Walker, was designed to give each pitcher five days of rest before their first regular-season appearance. If deGrom isn't ready for Opening Day, projecting the rotation might not be as simple as moving each pitcher up one day.
"I'd really like to stay away from changing four guys' plans that were carefully scripted to begin the season with an extra day's rest going in," Showalter said.
Should deGrom miss meaningful time, right-hander Tylor Megill or left-hander David Peterson are the most likely options to join the Mets rotation. Megill went 4-6 with a 4.52 ERA in 18 starts last year -- his lone major league season. In 25 appearances (24 starts) across the past two seasons, Peterson is 8-8 with a 4.64 ERA.
DeGrom, a two-time Cy Young Award winner, got off to a sensational start last season, going 7-2 with a 1.08 ERA over 15 outings. But an elbow injury sidelined him the entire second half.
The right-hander reported to camp healthy this year and has permitted one run over five innings in Grapefruit League games, striking out 10. His most recent outing was Sunday against the Cardinals.
Earlier in camp, deGrom said he plans to opt out of his contract after this season and become a free agent.
The Associated Press contributed to this report.