Mickey Rourke -- The Wrestler Becomes ... THE MMA FIGHTER!

Mickey Rourke played a fake fighter in "The Wrestler" -- but he was scrappin' FOR REAL this week during a hardcore MMA training session in L.A. with Jay Glazer.

The "NFL on Fox" star hit the mats with Rourke at his "Unbreakable Performance" MMA gym -- the place he co-owns with retired NFL star Brian Urlacher.

Glazer says Rourke's been "training his ass off" ... though it's unclear if he's doing it for fun -- or if he plans to actually compete ... which is unlikely since he's 62-years-old.

FYI -- Rourke is no stranger to combat sports ... he famously boxed back in the '90s after he had already established himself as a Hollywood star.