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PORT ST. LUCIE — Frankie Montas’ triceps soreness following his first bullpen session last week has turned into a rotation vacancy for the Mets to begin the season.

The right-hander is dealing with a high-grade lat strain, manager Carlos Mendoza said Monday, and has been shut down from throwing for 6-8 weeks.

Once Montas begins throwing again he will need a full spring training buildup, meaning in a best-case scenario he won’t return to the Mets until at least mid-May.

The 34-year-old Montas returned to New York, where he was scheduled to receive a platelet rich plasma (PRP) injection.

Mendoza isn’t backing off his plan to utilize a six-man rotation, leaving Tylor Megill and Paul Blackburn among the primary internal options to replace Montas.

Kodai Senga, Sean Manaea, Clay Holmes, David Peterson and Griffin Canning are the projected top five with Montas sidelined.

Montas, who arrived over the winter on a two-year deal worth $34 million that contains an opt-out after the first season, dealt with a shoulder injury that limited him to one appearance for the Yankees in 2023.

He returned to start 30 games last season for the Reds and Brewers and pitched to a 4.71 ERA over 150 ²/₃ innings.

Mendoza said he’s “pretty confident” in the club’s depth options to replace Montas in the rotation.

“There’s a reason why they are here,” Mendoza said. “We know that over the course of 162-plus [games,] we are going to need 8-10 guys. It’s nothing new for us. Guys will step up, guys will get opportunities and we feel good with the options that we have.”

Jose Quintana, who spent the past two seasons with the Mets, is among the top pitchers remaining in free agency.

The Mets also could still try to reignite talks with the Padres for a trade that would bring Dylan Cease to Queens.

There hasn’t been any thought, according to Mendoza, of moving Jose Buttó into the starting rotation. Buttó began last season as a starter before thriving as a reliever in the second half.

The Mets want to keep the right-hander in a multi-inning relief role, at least at this point.

Spring training injuries to pitchers aren’t unusual. Last season the Mets lost Senga to a shoulder strain early in camp, and he didn’t pitch for the team until July.

A year earlier Quintana had rib surgery in spring training that sidelined him for the first half of the season.

In 2022, the Mets lost Jacob deGrom in spring training to elbow discomfort. He also missed the first half of the season.

Megill has been a valuable replacement starter during much of that stretch.

The right-hander rebounded from a shaky first half last season and was strong during the stretch run, pitching to a 2.32 ERA in his final six starts for the club.

“The pitch count was way lower,” Megill said last week. “Finishing up the fifth and then being able to go into the sixth and finish the sixth inning. That has always been my problem, the fifth and sixth innings. When I came back, the fifth and sixth innings were completely different.”

Blackburn, who arrived from the A’s at last year’s trade deadline, is returning from cerebralspinal fluid leak repair surgery that was performed in October.

“We’re building him up to be a starter, so we’ll see where we are at the end of camp,” Mendoza said. “It all depends. But as of right now everything is progressing in the right direction. Our goal is to have him, just like the other guys.”