UFC's Derrick Lewis Destroys Trash-Talking Boxer In Gym Fight

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.