New York Mets pitcher Kodai Senga during a spring training...

New York Mets pitcher Kodai Senga during a spring training workout on Feb. 14, 2024, in Port St. Lucie, Florida. Credit: Newsday/Alejandra Villa Loarca

PORT ST. LUCIE, Fla. — Kodai Senga, the Mets’ best pitcher and presumed Opening Day starter, will miss the start of the season because of a strained right shoulder, the team announced Thursday.

The Mets offered no timeline on when he might return, but president of baseball operations David Stearns described the injury as a “moderate” strain.

Their immediate course of treatment: a shutdown “until these symptoms subside and strength returns to his normal levels,” Stearns said. Then Senga will have to go through his own spring training ramp-up, which is at least six weeks.

An outside expert suggested such a shutdown could last at least a couple of weeks and potentially much longer.

“How long exactly [Senga will be out], I can’t tell you right now,” Stearns said. “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. This is not a surgical-type problem. This is something with rest and treatment, potentially an injection, that can move forward.”

Senga said through an interpreter: “The message was it's not anything severe at this point, and if we can treat it now — there aren't any effects on other parts of the body — we will be able to get over this hump quick.”

His sudden indefinite absence created a competition for an open rotation spot. However, the Mets do not expect to add a starter because of his development, according to Stearns.

The primary fill-in candidates: Tylor Megill, Joey Lucchesi and Jose Butto. Stearns said the Mets prefer that their yet-to-debut prospects — Mike Vasil, Christian Scott, Dominic Hamel — get more experience in the minors.

“We’ve got a lot of exciting young starters in camp, so I’m excited to watch those guys as well,” Stearns said. “We’re going to ask people to step up. That’s what happens over the course of a baseball season. We knew we were not going to go through a full season with just five or six starters. And so here we are.

“We’ve got plenty of options. I’m looking forward to watching that competition in camp. I’m confident that we’ll have guys step up.”

Nothing wrong with Senga popped up when pitchers and catchers took their start-of-camp physicals on Feb. 13, according to Stearns.

But a day after Senga tossed his first bullpen session, he told team officials he didn’t recover the same way he usually does. That is a normal occurrence this time of year, so the Mets didn’t think much of it.

When it happened again this week, they had Senga get an MRI.

The diagnosis came Thursday morning: a right posterior capsule strain in the back of his shoulder.

“My understanding is the anterior capsule, the front of the shoulder, is the one where it can be bad,” Stearns said. “The posterior capsule, the back of the shoulder, less bad. And in this case, look, he did the right thing. He told us. We were able to get it looked at, begin treatment and hopefully we caught it early enough where this is just a speed bump.”

Dr. Laith Jazrawi, the chief of the division of sports medicine at NYU Langone Orthopedics, who has not treated Senga, said a capsule strain often requires a two-week rest period. That the Mets called it “moderate” indicated to Jazrawi that it might be more in the range of 4-6 weeks.

Mets officials emphasized that they don't know how long it will take for Senga to feel normal.

“It is unusual to have a strain like that,” Jazrawi said. “You usually see that with someone who is overthrowing, throwing very hard, or someone who is just overusing it, throwing too much.”

Senga said he experienced a version of this about five years ago after transitioning from relief pitcher to starting pitcher in Japan.

“Out of experience, not being at 100% definitely puts a hamper on my performance,” he said. “If I was asked, can you throw 96, 97, 98? Yeah, I can throw 96, 97, 98. But the game isn’t about throwing fast. It’s about getting hitters out. And I felt like at this point, I’m not going to be able to perform at the highest level, so giving a little bit of time is the right move.”

This re-raises questions surrounding Senga’s schedule when he returns.

Last season, the Mets massaged their rotation around giving him extra rest to better simulate the once-a-week schedule he had in Japan. It worked: He had a 2.98 ERA and 1.22 WHIP in 29 starts (just three on the normal four days of rest).

The Mets had indicated they’d been planning more of the same for 2024.

“It's not really up to me, but 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. “But that's not the case. I'm not going to be pitching on Opening Day. I'm not going to be pitching when the season starts. And the reason why I'm giving myself a little bit of time is so I can come back into the season and I'm able to pitch on four days rest or five days or whenever they need me.”

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