Mets' J.D. Martinez Breaks Willson Contreras' Arm Swinging At Pitch

J.D. Martinez broke Willson Contreras ' arm while swinging at a pitch Tuesday ... and if you're squeamish, you might want to skip this one.

The unfortunate scene played out in the second inning of the Mets' clash with the Cardinals at Busch Stadium, when Martinez attempted to put his barrel on a Miles Mikolas slider.

The New York designated hitter, though, ended up making contact with Contreras' forearm instead -- and replays showed once the bat hit the catcher, a bone appeared to break instantly.

Contreras writhed around on the ground in a heap of pain, before trainers raced to his aid. Eventually, he was hauled off the diamond and into a doctor's office ... where he was diagnosed with a fracture.

He's now expected to require surgery that will keep him out of St. Louis' lineup for the next 6-to-8 weeks.

"Once I tried to do some motion stuff," Contreras said of the injury after the game, "there was some cracking in [the arm] and I knew it was bad."

Martinez -- who was actually awarded first base on the play due to catcher's interference -- said he felt awful over the incident ... adding that he had zero intention of hitting his opponent.

It's obviously a tough blow for the Cards -- Contreras had been their best hitter, batting .280 with six home runs this season. And, to make matters, worse, they ended up losing the game, 7-5.

Talk about a tough day at the ballpark.