Justin Bieber -- 'Crazy Athletic, Real Potential' ... Says Ex-Boxing Champ

Justin Bieber's no bitch in the ring -- the guy's got REAL BOXING POTENTIAL ... so says former champ Andre Berto who says Bieb could be a real contender if he takes the sport seriously.

We spoke to the former welterweight champ -- who trains at the same L.A. boxing gym where Bieber worked the mitts with Floyd Mayweather earlier this week -- who says he was legitimately impressed.

"The kid is CRAZY athletic," Berto says ... " I seen that from kickin' it with him a few years back. He has good speed and power."

But Berto has a strong warning for the pop star -- it's no joke.

"This boxing game is nothing to play with. It's not like dancing or singing. If you get your ass CRACKED it’ll make you change your whole perspective on things."