PORT ST. LUCIE, Fla. — For the third spring in a row, infielder Brett Baty is fighting for a spot on the Mets’ opening day roster.

On Thursday, Baty’s chances of making the roster improved after it was revealed that second baseman Jeff McNeil was out at least three weeks with a low-grade oblique strain. The 25-year-old was quickly deployed against the Boston Red Sox, where he was quickly tested, and then again against the St. Louis Cardinals on Friday at Clover Park.

“It’s got to be a next man up mentality,” Baty told reporters as he walked off the field during batting practice on Thursday. “Injuries are part of the game, and if that means I need to get more reps at second base, then that’s what that means. I’m all for it.”

Baty’s a natural third-baseman. He told NJ Advance Media that before last season when he played 27 games (217 innings), the last time he played second base was “in little league a little bit” — that’s hardly enough time to learn the position.

While he’s played his entire career in the infield, the job of a second baseman is a lot different than at third.

“At third base it’s more angles and catching the ball, throwing across the diamond,” Mets manager Carlos Mendoza said. “Whereas a second baseman is involved in a lot of team defenses, bump plays, covering first base, getting familiar with the first baseman … There’s a log going on that only game reps will teach him.”

Baty adds another challenge to learning his new position.

“Just knowing where to go on the cuts and relays and stuff like that,” Baty told NJ Advance Media. “I’ve always played third base, so it’s second nature at this point. … At second base, I need to be conscious of (where to be).”

In one relay opportunity against Boston, Baty was exactly where he needed to be. Not because he had practiced it, but because of his natural instincts.

“It was like a ball hit down the line and I was like the double-cut man,” Baty said. “Kind of second nature. Didn’t even think about it. Just ran over there.”

The Mets have 10 Grapefruit League games left, including split squads. His bat is proving him worthy of a big league spot — he’s batting .375 with a 1.100 OPS — but his chances to prove he’s worthy of being the starting second baseman are quickly running out.

“I think all the reps I can get are going to be good no matter where they are,” Baty said.