Mets outfielder Michael Conforto during a spring training workout on...

Mets outfielder Michael Conforto during a spring training workout on Feb. 20 in Port St. Lucie, Florida. Credit: Newsday / Alejandra Villa

PORT ST. LUCIE, Fla. — Michael Conforto’s uphill battle to make the Opening Day roster ended on Sunday morning when he was told he’ll be staying behind when the Mets break camp on Monday.

Conforto will start the season on the 10-day disabled list. He still is way ahead of schedule, which originally had him returning around May 1 after shoulder surgery.

Conforto, who made his spring training debut on Friday, will get consistent at-bats in minor-league spring training games. The Mets, who will open the season against the Cardinals on Thursday at Citi Field, can activate Conforto as early as April 5.

“We just really felt after talking about it over and over that he needs to get out there and play seven, eight, nine innings multiple times before we throw him out there,” manager Mickey Callaway said. “I think we considered at one point maybe having him off the bench, but he’s so important to us, he needs to be ready to play nine innings when he is with us and play every day.”

Conforto will be joined on the DL by lefthander Jason Vargas, who underwent surgery on Tuesday to remove part of a broken bone in his right (non-pitching) hand. Vargas has continued to throw and will be activated once he can put a glove on and pitch without fear of further injury.

Also starting the season on the DL: David Wright. The captain plans to be on hand for the opener. He is not supposed to do any baseball activity until mid-May.

With the fates of Conforto and Vargas decided, the 25-man roster took shape. Seth Lugo will be the fifth starter until Vargas returns. It’ll be Lugo’s first Opening Day roster spot.

“It’s pretty cool,” he said. “I am a big-leaguer.”

Brandon Nimmo and Juan Lagares will share centerfield until Conforto is ready. Infielder/outfielder Phillip Evans, who has added third catcher to his resume, made the team as the final position player.

Evans said he officially was told at 6:30 Sunday morning when he was getting in the hot tub in the training room. So you can make the club from the tub.

Callaway is going with a 12-man pitching staff. The seven-man bullpen has righthanders Jeurys Familia, AJ Ramos, Anthony Swarzak, Robert Gsellman, Paul Sewald and rookie Jacob Rhame and lefthander Jerry Blevins.

On Sunday against the Marlins, Callaway trotted out what very well could be his Opening Day top eight against Cardinals righthander Carlos Martinez. It was Nimmo, Yoenis Cespedes, Jay Bruce, Todd Frazier, Asdrubal Cabrera, Adrian Gonzalez, Kevin Plawecki and Amed Rosario.

The best news of all could be how Cespedes looks. He hit his sixth home run in 12 at-bats since returning from a cortisone shot in his right wrist and threw out a runner at the plate.

As a reminder of what can happen in the last days of spring training, Nimmo fouled a ball off his right knee area in the fifth inning. He remained in the game.

“It hurt like when you feel your funny bone,” said Nimmo, who fouled a ball off the same spot last week.

The Mets will play a modified 6 1⁄2-inning game against their Triple-A Las Vegas affiliate here on Monday before heading to New York. Major-leaguers will pitch for both teams, with Matt Harvey and Jacob deGrom starting (and throwing only 10 pitches per inning) and the entire seven-man bullpen getting some work.

The Mets finished the exhibition season with a 10-18-3 record after Sunday’s 4-3 win over the Marlins. It was Callaway’s first spring training as a manager, and by all accounts, it went pretty well.

Said Callaway, “We’re looking forward to a good season.”

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