Mickey Rourke may be the world's biggest defender ... he will kick your ass if you talk smack. 

Mickey was leaving Bootsy Bellows Friday night in WeHo, and he didn't take kindly to some negative comments about the guy he clearly believes is the greatest rapper of all time

Rourke was super tight with Tupac back in the day -- to the point he knew the location of his bullet wounds. Did you know 'Pac was shot in the balls? Now you do.