Clay Holmes, who has added two new pitches, called it...

Clay Holmes, who has added two new pitches, called it “definitely an honor’’ to start on Opening Day. Credit: Newsday/Alejandra Villa Loarca

PORT ST. LUCIE, Fla. — Clay Holmes, who struggled to such a degree last season that the Yankees removed him from the closer role amid their playoff push, will be the Mets’ starting pitcher on Opening Day.

When the Mets play the Astros in Houston on March 27, Holmes will make his first start since he was a Pirates rookie in 2018. It will be his fifth start in the majors overall.

That the Mets selected Holmes, signed during the offseason to a three-year, $38 million contract with the idea of trying to turn him into a starter, speaks to how well that transition has gone — and to the uncertain state of the Mets’ rotation overall.

“It’s definitely an honor. It’s been a crazy few months,” Holmes said. “But just a testament to obviously the work I’ve put in [as well as] the people I’ve been around here from the organization. There was a plan in place from the get-go. I believed in it. They believed in me. And there’s been a lot of work these past few months. This is just a step.”

Manager Carlos Mendoza said: “He earned it. We like what we’re seeing. I know it’s a small sample size here, but it’s very encouraging. He really put himself in a really good position — showed up here early in camp, he was ahead of everyone. It’s more the way he’s looked so far making that transition, we’re comfortable with him going on Opening Day.”

Among the more obvious candidates to draw the season-opening assignment, Kodai Senga “won’t be ready” for the first game,” Mendoza said. But the Mets do expect him to start one of the initial five games against the Astros or Marlins.

Sean Manaea, the Mets’ best starter in 2024, is out for at least the first few weeks because of a strained right oblique.

That left Holmes and David Peterson as options. Filling out the rotation will be two of Griffin Canning, Paul Blackburn and Tylor Megill.

Holmes’ emergence as a potentially legitimate starter is “super-important,” Mendoza said, given injuries to Manaea and Frankie Montas (right lat strain).

Heading into his fourth Grapefruit League outing Saturday, Holmes has thrown  9 2⁄3 scoreless innings with 13 strikeouts (to four walks).

Among the subplots within this subplot: He has added a changeup, which has drawn rave reviews from teammates and coaches, and a cutter to go with his usual sinker/slider/sweeper combo.

“I don’t know if we can put too much on the results in spring training, but the characteristics of his pitches, the metrics, how he is feeling, the workload and things like that [are encouraging],” Mendoza said.

When Mendoza and pitching coach Jeremy Hefner informed Holmes of the news Thursday, his reaction was “typical Clay Holmes,” Mendoza said. That means he didn’t have much of an outward reaction at all.

“He doesn’t get too high, he doesn’t get too low,” said Mendoza, who initially got to know Holmes when he was the Yankees’ bench coach through 2023. “I know it means something — it’s the Opening Day start. But knowing Clay, when he’s going really good, he’s the same guy. When it’s hard for him, he’s the same guy.”

Holmes said: “I’m not a super-emotional guy. I wasn’t doing jumping jacks, but I’m excited. I may not always show it. It wasn’t a goal coming into this . . . but to have a little bit of, I guess, you could call it a reward for the work I’ve had to put in, it feels good.”