Buster Douglas -- the man who SHOCKED the world and beat Mike Tyson -- tells TMZ Sports ... Andy Ruiz's TKO of Anthony Joshua is one of the biggest upsets in boxing history.
Bigger than his 1990 upset of undefeated heavyweight champ "Iron" Mike?? Buster -- a 42-1 underdog when he KO'd Tyson in the 10th round -- wouldn't pick which was bigger ... but was clear, this fight was historic.
"I think it was a great fight, man. It was a big upset.