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.