Chad Johnson is taking his celeb fight SO seriously, he's getting in the ring with the Charlo twins to prep for the tilt ... and he tells TMZ Sports the guys are whoopin' his ass!!
The ex-NFL superstar -- who's set to square off with MMA fighter Brian Maxwell on the undercard of the Mayweather-Paul fight on June 6 -- broke it all down for us this week ... explaining he's sparring Jermell and Jermall Charlo to get ready for his match.