In a statement on Friday, the Mets said deGrom won't throw for up to four weeks and then will be reevaluated.
DeGrom, 33, is widely regarded as the best pitcher in baseball. In 15 starts last season, he posted a 1.08 ERA and in 92 innings struck out 146 batters and walked just 11. But a variety of injuries caused him to miss starts, and elbow pain sidelined him for the entire second half.
The scapula, or shoulder blade, is a vital bone in the pitching process. Stress reactions, which are injuries to bones typically brought on by repeated movement, are rare in pitchers' shoulders. Typically, stress reactions are healed through rest.
DeGrom, who won back-to-back National League Cy Young Awards in 2018-19 and finished ninth in voting last season even with only a half-season's worth of games, was expected to play a significant role in the Mets' efforts to unseat the defending World Series champion Atlanta Braves, who have won four consecutive NL East titles.
New York signed former Washington Nationals ace Max Scherzer to a three-year, $130 million contract this winter to pair with deGrom atop its rotation. Scherzer is now expected to start Thursday against the Nationals on Opening Day.
In his two starts this spring, deGrom had sparkled, giving up one run in five innings and striking out 10 without walking a batter. He felt tightness in his right shoulder Thursday, prompting the team to seek an MRI of deGrom's arm Friday morning.
While the shoulder -- a tangle of overlapping muscles -- looked clean, the bone prompted enough concern to shut down deGrom for the start of what could be his final season with the Mets. He has said he plans to opt out of his contract following this season, during which he'll be paid $33.5 million, but added that he "love[s] being a Met" and that "it would be really cool to be one for my entire career."
Chosen in the ninth round of the 2010 MLB draft by New York, deGrom has a career record of 77-53 with a 2.50 ERA and 1,505 strikeouts in 1,261⅔ innings.