Bad news for Manny? "Floyd going to f**k buddy up," Broner says.

Manny-Floyd rematch talks have been swirling since the second the final bell rang after Pacquiao's somewhat surprising win over Keith Thurman last Saturday.

In fact, Floyd and Manny have already begun beefing on social media ... trading serious barbs on Instagram and Twitter on Tuesday.

Broner says the rematch is definitely going to happen ... telling us when we got him out at Tao in L.A., "They'll fight again. That's why they doing all this cappin'. They'll fight again."

But, the stud boxer sees bad news for Manny in the rematch, saying, "Floyd going to beat his ass again ... Floyd going to f**k him up again and they both going to retire."

Of course, Broner just fought Manny and lost back in January ... so he's clearly got some insight into the fight -- and he says he has no doubts Floyd will repeat the beating he put on Pacquiao back in 2015 in the run back.