Showalter had said Thursday that deGrom felt tightness in his right shoulder while throwing long toss, then on Friday said that the right-hander would not play against the St. Louis Cardinals.
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.
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.