Roy Jones Jr., listen up ... here's some sage boxing advice from Rocky himself -- DON'T GET HIT BY MIKE TYSON!!!!
TMZ Sports got Sylvester Stallone leaving Fia in Santa Monica this week ... and when we asked about RJJ's chances in his November exhibition with Iron Mike -- Sly didn't love them.
In fact, Stallone seemed straight-up concerned for Roy!!!
"Well, let's put it this way," Stallone said. "What do you think is going to work? A 22 against a 45?