Sylvester Stallone -- Mayweather Will Stay Undefeated … Unless (VIDEO)

Sylvester Stallone knows the secret to defeating Floyd Mayweather Jr. … but it doesn’t have anything to do with his next opponent, Andre Berto.

Almost no one is giving Berto a chance to win the fight -- so we asked Sly if he thinks he's even got a puncher's chance to upset Money. Check it out ... Sly says there's a chance, but not a puncher's one.

Stallone also throws a playful jab at "Southpaw" star Jake Gyllenhaal's on the topic of big screen boxers. We'll give you one guess which actor/fighter he IS backing.