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.