PORT ST. LUCIE, Fla. -- The nightmare scenario for the Mets, entering this spring, was an injury to pitcher Kodai Senga. In many ways, Senga is their least replaceable player, an ascending star who opened camp well-qualified as their ace.

Barely a week into Spring Training, Senga now qualifies mostly as another question mark. He will miss Opening Day due to a shoulder strain and begin the season on the injured list, president of baseball operations David Stearns said Thursday, costing the team its most important rotation member for an indefinite period.

“It is regretful that I’m not able to be a part of it in the beginning of the season,” Senga said through an interpreter, “but I think I just have to be positive and look forward to being back out there with the team.”

An MRI taken Wednesday revealed a moderate right posterior capsule strain for Senga. The Mets intend to shut him down from throwing until his symptoms subside and his strength returns to normal levels.

“How long exactly, I can’t tell you right now,” Stearns said, adding that Senga will not require surgery.

“What I can say at this point, comfortably, is we don’t expect Opening Day. But I do expect him to make a bunch of starts for us this year.”

Senga, who is in the second season of a five-year, $75 million contract, began having trouble recovering from his side sessions in the early days of camp. He alerted Mets trainers to the issue multiple times, and after the second of those conversations, team officials sent him for the MRI. That revealed the capsule strain.

“He did the right thing. He told us,” Stearns said. “We were able to get it looked at, get him treatment, and hopefully we caught it early enough that this is just a speedbump.”

Rather than bemoan the issue in the immediate aftermath of the news, Mets officials painted it as optimistically as possible. In addition to Stearns’ comments, manager Carlos Mendoza said he felt “a little bit better” after hearing the diagnosis on Senga, because “every time you send [a player] for an MRI, you’re expecting the worst.”

“In this case, we got a little bit of good news, right?” Mendoza said.

Still, the Mets will be without their top starter at least for Opening Day and potentially for longer. The Mets’ most effective starter last season, Senga went 12-7 with a 2.98 ERA as a rookie, finishing seventh in National League Cy Young Award voting. Those numbers included a 2.44 ERA with 101 strikeouts and a reduced walk rate over his final 14 starts, giving the Mets optimism that he could become an even more accomplished pitcher in his sophomore season.

But injury concerns have dogged Senga since the early days of his Mets tenure, when the New York Post first reported that the team signed him despite an “iffy” medical review. Given Senga’s history as a once-per-week starter in Japan, team officials drew up a plan last year to pitch him every six days instead of every fifth game, frequently rejiggering their rotation to accommodate him. He made only three starts on regular rest, prompting the Mets to use a six-man rotation on multiple occasions.

Although Senga was somewhat uncomfortable with the arrangement, he abided by it and finished the season healthy, throwing 166 1/3 innings over 29 starts. Early this spring, Mets officials were already discussing ways to incorporate a sixth man into their rotation again.

“I guess if I was in the rotation now and they asked me, ‘Do you prefer a five-man or a six-man?’ I might say six-man,” Senga said, noting that he’s had no issues with his elbow since coming to the States. “But that’s not the case. I’m not going to be pitching Opening Day. I’m not going to be pitching when the season starts. The reason why I’m giving myself a little bit of time is so that I can come back into the season and I’m able to pitch on four days’ rest or five days’ rest or whenever they need me.”

Stearns confirmed that with Senga sidelined, Tylor Megill, Joey Lucchesi and José Butto are the most likely candidates to take his rotation spot. The Mets have another starter with some MLB experience, Max Kranick, in camp, as well as three high-profile prospects in Christian Scott, Mike Vasil and Dominic Hamel. The latter pitchers are not in serious roster contention at this point but could become possibilities early in the regular season.

“We’ve been talking about our depth a lot, and here we are,” Mendoza said. “We’re going to get it tested. They’re going to get an opportunity. We’ll see how the camp plays out.”