Demetrious Johnson thinks Floyd Mayweather would be EASY WORK in the octagon -- telling TMZ Sports he'd obliterate the boxing superstar if the two clashed in the UFC.
After Dana White confirmed he was talking with Floyd about a "UFC deal" ... DJ immediately threw his name in the mix as a possible first opponent.
In fact, Mighty Mouse joined the guys on the "TMZ Sports" TV show (airs weeknights on FS1) and broke down exactly how he'd punish the 40-year-old.