JUPITER, Fla. — Entering the 2022 season with a record payroll and several new talented players, the Mets had high hopes. Perhaps the biggest reason for that revolved around star pitchers Jacob deGrom and Max Scherzer, the leaders of what was supposed to be one of the best starting rotations in baseball.

But six days before opening day in Washington on April 7 — an assignment that was supposed to go to deGrom, a two-time Cy Young Award winner — the Mets are reeling. On Friday, a day after the team first learned its ace was dealing with discomfort in his throwing shoulder, the Mets announced that he may miss all of April.

The diagnosis after a magnetic resonance imaging examination on Friday morning: a stress reaction (considered a precursor to a stress fracture) in his right shoulder blade that has caused inflammation. The Mets said that deGrom will not throw for up to four weeks, then will have another M.R.I.

“He’s disappointed,” Mets General Manager Billy Eppler said as his team played the St. Louis Cardinals during a spring training game in Jupiter, Fla. “We’re disappointed.”

DeGrom’s injury comes at a critical time not only for the Mets but for himself. After winning the National League Cy Young Awards in 2018 and 2019, deGrom appeared on track to claim another in 2021. But after keeping his earned run average to 1.08 over 92 innings by July, deGrom missed the final three months of the season with a right elbow injury — a scary proposition for any pitcher but particularly for one who had Tommy John surgery in 2010 and a separate surgery to repair nerve damage in his elbow in 2016.

DeGrom, 33, reported to this spring training healthy and, clearly with the goal of having a dominant season, announced that he planned to exercise the clause in his five-year, $137.5 million contract extension that allowed him to opt out after the 2022 season.

Asked how much time he expected deGrom to miss, Eppler said it was hard to tell and the timeline depended on deGrom’s healing. Eppler said deGrom could feel better in less than four weeks, but based on what the M.R.I. revealed, a month was a reasonable period to wait before undergoing another.

Should deGrom resume throwing in four weeks, he would still need to get back into game shape. A guideline for starting pitchers is that every week missed requires another week to build the arm and body back up, so deGrom could be facing at least two months away.

Stress reactions in the throwing shoulder have previously bedeviled other pitchers, such as Michael Wacha and Brandon McCarthy, sometimes repeatedly. Reacting to the news of deGrom’s injury, the retired McCarthy wrote on Twitter, “Since this injury technically should be named after me I can say that this really isn’t a big deal. Not as big as it seems. He’ll be just fine.”

Eppler said deGrom’s injury developed over time and that doctors found that the rest of the pitcher’s shoulder was sound. He and Mets Manager Buck Showalter said they had no idea if this year’s delayed and truncated spring training led to deGrom’s injury.

“It’d be an easy blame,” Showalter said, then said of other injuries across baseball, “We’re all operating under the same one. You see a lot of them going on.”

Even though Scherzer, 37, a three-time Cy Young Award winner the Mets lured this winter with a record a three-year, $130 million deal, seemed like the obvious choice to replace deGrom on opening day, Showalter said he wanted to talk with other members of the front office before announcing a decision.

Among the less proven options to fill the hole in the rotation: Tylor Megill, David Peterson and Trevor Williams. Eppler said he felt good about the rotation depth the Mets had already amassed. After Scherzer in the rotation, the Mets have veteran pitchers Carlos Carrasco, Taijuan Walker and Chris Bassitt, whom they acquired in a trade last month.

“The sky’s not falling,” Showalter deadpanned at the end of Friday’s game, which was briefly interrupted by the wet weather. “It’s raining.”