If ya wanna know how Floyd Mayweather vs. Manny Pacquiao II will go down, just watch the first time they fought in 2015 ... 'cause Andre Berto tells TMZ Sports he's expecting similar results when they duke it out again later this year.

49-year-old Floyd and 47-year-old Pacquiao's professional bout in Las Vegas is just months away ... so we had to get Berto's take on the spectacle.

Even though Pac-Man is active and Money has been retired for years, Berto doesn't expect "explosive" shots from either guy ... which would line up with the snooze fest their first go-round ended up being.

He did say he thinks they will still put on a show, even though both legends are past their prime ... 'cause Pacquiao desperately wants that win.

Berto gives the edge to Floyd ... 'cause he's got the knowledge and experience no one else has.

Berto believes Floyd HAD to put his perfect record on the line in order to make the fight enticing for boxing fans ... but like he said, he doesn't expect it to be ruined after September 25.