First he beat up his opponent ... then Derrick Lewis needed to beat up the toilet!
The Black Beast dropped another gem during his post-fight interview Saturday night ... saying, "I gotta take a sh*t" -- ON LIVE TV!
35-year-old Lewis had just obliterated #10 ranked heavyweight Aleksei Oleinik in the 2nd round of his main event fight ... when the post fight interview with Paul Felder began.
Lewis apparently didn't realize he was LIVE -- because he turned to someone off-camera and revealed he had some junk in his trunk.