Stallone, 63, who performed his own stunt work on the set of the movie, was left reeling in pain after one fight with wrestling star Stone Cold Steve Austin turned so "vicious" he had to undergo corrective surgery - and now has a plate in his neck.
He tells Britain's FHM magazine, "It was seven guys, kicking each other's a**, one guy tougher than the next. No joke, our stunt guys were begging for mercy.
"My fight with Stone Cold Steve Austin was so vicious that I ended up getting a hairline fracture in my neck. I'm not joking. I haven't told anyone this, but I had to have a very serious operation afterwards.
"I now have a metal plate in my neck."
Stallone stars alongside fellow tough guys Bruce Willis, Jet Li, Mickey Rourke and Jason Statham in the movie, in which a group of mercenaries attempt to overthrow a South American dictator.