NEW YORK — Mets outfielder Starling Marte’s bad knee “flared up” in the bottom of the ninth inning against the Yankees, manager Carlos Mendoza revealed on Sunday.

On the fourth pitch of an at-bat against Yankees closer Devin Williams, a high changeup, Marte motioned as if about to swing, but promptly stopped. For a brief second, he seemed to buckle on his left knee and could be seen grimacing.

Stepping off, he bent forward, massaging his knee, and was quickly joined by the team’s trainers. Eventually, he completed his at-bat, grounding out to third base

“That same knee that he had issues with last year (and) spring training,” Mendoza said after the Mets’ 6-4 loss. “They’re taking a look at him right now and we’ll see what we’re dealing with here.”

Marte has been dealing with what the Mets called a “bone bruise issue” back in spring training. At the time, Stearns gave some insight into a conversation he had with Marte regarding playing through pain.

“He told me, early in camp, ‘I’ve played through pain my entire career. I’m used to it. I’ll keep doing it,‘”Stearns told reporters in March.

How serious do the Mets believe Marte’s injury is?

“I’ve got to wait,” Mendoza said. “I was just talking to him, and he was like, ‘Obviously, I felt it.’ But, I’ve got to wait and see what the doctor says.”

Marte has played an important role for the Mets this season.

Initially, he was expected to lose playing time after the Mets signed Juan Soto. But he embraced the role, seeing it as an opportunity that makes him “stronger” because it allows him to help his teammates in a different way.

Since outfielder Jesse Winker landed on the injured list with an oblique strain in early May, Marte’s returned to an everyday player and is thriving.

He’s 22-for-63, a .349 batting average in his last 17 games, including a three-hit effort against the Yankees on Sunday.