Anthony Joshua's gotta be kicking himself this morning after losing in one of the biggest upsets in boxing history -- and he's got no one to blame but himself ... and possibly Drake.
AJ lost his 3 heavyweight titles in Madison Square Garden Saturday night to Andy Ruiz Jr. -- who was an 11-1 underdog -- by TKO in the 7th round. It's being called the biggest upset since Mike Tyson lost to Buster Douglas in 1990 ... but we all should've seen it coming when Joshua cozied up to Drake.