"Joshua, I think he's a broken man. Mentally, he's done."
That's Deontay Wilder saying Andy Ruiz didn't just beat Anthony Joshua in the ring ... he crushed the man's soul.
Wilder touched down at LAX on Wednesday night and couldn't wait to GO IN on his heavyweight rival -- explaining why Ruiz exposed AJ as a weak-minded fighter who can't hang with the best.
"When you perform that way as a champion??? He gave up!! He quit!!!!"
Of course, Ruiz -- a heavy underdog -- beat the snot out of Joshua at Madison Square Garden on Saturday .