J.D. Martinez knew something was wrong when he hit Willson Contreras

ST. LOUIS — J.D. Martinez didn’t need to see the catcher’s reaction to know something was wrong.

Swinging at a pitch in the second inning on Tuesday, the Mets DH connected with Willson Contreras’ left arm. Contreras crumpled over in agony before leaving the game.

The Cardinals announced that Contreras’ arm was fractured.

Martinez was awarded first base on a catcher’s interference on the play.

“You are expecting to hit a ball and you hit an arm and you are like, ‘What is that?’ ” Martinez said after the Mets’ 7-5 victory at Busch Stadium. “I hit meat. I didn’t hit just a glove where you kind of just point back at the catcher. It was just solid. I hit him good. When I got to first base I felt terrible.”

Though Martinez stands deep in the batter’s box and interference plays are on the rise as catchers move closer to the plate for framing purposes, Martinez said connecting with Contreras was the “last thing” he would have expected.

Kodai Senga will throw a bullpen session before a decision is reached on his next step.

There was the possibility the right-hander would be thrust into a minor league rehab assignment later this week after completing a third session facing hitters over the weekend. But Senga, according to manager Carlos Mendoza, prefers to first throw a bullpen session.

“Physically he is feeling well,” Mendoza said. “I think it’s just more mechanics that he wants to throw a bullpen to work on some of the things he’s been working on. But as far as how he’s feeling, it’s all good news.”

Mendoza said it’s possible Senga would face hitters — instead of making a rehab start — after throwing his bullpen session, which will occur Wednesday or Thursday.

Senga, who was diagnosed with a shoulder strain early in spring training, won’t return to the Mets until at least early June.

Tylor Megill pitched four shutout, hitless innings for Triple-A Syracuse with seven strikeouts and one walk in his third rehab start.

The right-hander is scheduled to pitch again in a rehab start on Sunday for Syracuse, according to Mendoza.

Drew Smith is scheduled to pitch an inning for Syracuse on Wednesday.

It will be the right-hander’s first rehab appearance since he was placed on the IL on April 27 with right shoulder soreness.