Gennady Golovkin thinks it's pathetic that Canelo Alvarez couldn't knock out a weak ass Julio Cesar Chavez Jr. ... and says he's in for a rude awakening when they face off in September.

Triple G joined the guys from "TMZ Sports" and went off on Canelo -- mocking everything about that garbage matchup back in May. 

GGG also clowned Canelo for standing up between rounds of the Chavez fight -- saying he was just trying to look tough to impress women.

Golovkin -- who's on his way to Kazakhstan for Expo 2017 -- did give Canelo one compliment ... saying he's the first "real champion" he's ever fought.