This was no Mayweather vs. McGregor ...
UFC superstar Derrick Lewis told a boxer to PUT UP OR SHUT UP during an argument about MMA vs. boxing ... and the results did not go well for DL's opponent.
The 34-year-old heavyweight says he was working out in the gym on Wednesday, when the other man said, "He will knock me out because I’m a MMA fighter."
Lewis, not one for boring debates, told the guy to strap on the gloves and get in the ring ... and the rest -- well, just watch.
Lewis beats the guy up so badly, he almost punched him THROUGH THE ROPES!
Of course, Lewis -- aka "The Black Beast" -- is known as one of the toughest dudes in all of combat sports ... beating guys like Francis Ngannou, Alexander Volkov, Travis Browne and Roy Nelson.
Think he's gonna back down from some no-name boxer in a gym fight?!?!?