Orlando Bloom has some serious muscle on his side -- UFC Hall of Famer Forrest Griffin who tells TMZ Sports he wants to TRAIN the actor how to fight so he can take Bieber the next time they face off.
The 35-year-old former Light Heavyweight champ says he watched the footage of Bloom and Bieber going at it in a fancy Ibiza restaurant and says he was unimpressed with Bloom's attack.
"He threw a weak-ass punch!"
But Griffin says he's a fan of Orlando's movies and wants to help ... saying, "He can come to Vegas anytime to train with me. If they ever fight again, I can show him some moves so he can win."
"But I gotta warn Orlando ... I think Floyd Mayweather Jr. is gonna train Bieber -- and that guy's a monster."
Still, Griffin says if the two stars do ever square up, Orlando would have the advantage -- "He's not big, but it looks like he has the reach. Then again, everyone looks big next to Justin Bieber."