PORT ST. LUCIE — Mark Vientos was squeezed from his likely DH role in recent days, and Sunday he lost his roster spot.

The Mets optioned Vientos to Triple-A Syracuse to begin the season, three days after acquiring J.D. Martinez on a one-year contract worth $12 million.

Martinez will spend at least the first 10 days of the season in the minor leagues building up, but Vientos still wasn’t deemed the right fit for the roster.

“As we looked at the roster construction, some of the matchups early in the season, we just thought this made the right sense both for [Vientos] from a playing-time perspective and our team at the major league level,” president of baseball operations David Stearns said.

Stearns described Vientos as “frustrated” after learning he would be headed to the minors.

“But he’s a professional and so I think he would have certainly preferred if he got the other news and stayed on the club, but I would expect frustration,” Stearns said. “Now his challenge is to turn that into motivation, everything he can do to get back, and I think he will do that.

“I still view Mark as an everyday player. He will be a major league player for us this year and he will be a major league player, I think, for a substantial part of the season, just not on Opening Day.”

In the minors Vientos will play mostly third base — a position at which he competed with Brett Baty in spring training.

“Defensively there is still growth,” Stearns said, referring to Vientos. “He knows that and has worked incredibly hard defensively and deserves a lot of credit for that. He’s a better defender than he was last year and I think with regular continued reps at third base he will continue to grow there.”

Vientos led the Mets with five homers in the Grapefruit League, but the Mets see a need for him to continue improving offensively, along with his defense.

Vientos stood to receive many of the at-bats as the DH before Martinez’s arrival.

“There’s a lot of talent there,” manager Carlos Mendoza said. “It comes down to him getting everyday reps and he’s going to get a lot of that in Triple-A.”

Ji Man Choi was also informed he won’t be on the Opening Day roster, but the veteran first baseman/DH has decided to remain with the organization at Triple-A, according to a source, rather than opt out from his contract.

Zack Short, a versatile infielder, was told he’s on the team.

DJ Stewart will head north with the club, but isn’t officially on the roster — the Mets are still monitoring who might become available from other camps before Thursday’s opener at Citi Field.

“[Short] can fill a number of different spots on our team, he has played all over the place,” Stearns said. “That right-handed complement we thought was a nice fit on our team. He’s proven that he’s a very good defender at multiple positions and we have seen that this spring. And I also think he’s working through some swing adjustments that frankly look pretty good right now.”

As for the bullpen, the Mets still have three relievers — Michael Tonkin, Sean Reid-Foley and Yohan Ramirez — for two spots.

Baty was told he was on the Opening Day roster after an offseason and spring in which he showed defensive improvement, according to Stearns.

“He’s not going to be perfect over there,” Stearns said. “We told him he’s not going to be perfect over there and that’s fine. When we’re not perfect over there, it’s, ‘Let’s move on to the next play.’ Having Zack [Short] and Joey [Wendle] on the roster as defensive-oriented players helps in a variety of places.”

Stearns, in his first comments since Martinez’s acquisition, said the decision to sign the veteran DH wasn’t based on anything he saw the Mets lacking in camp, but rather an opportunity to improve.

“This is more about J.D. and who he is and the track record and what he can bring to a team and a clubhouse,” Stearns said. “We are excited to have that.”