Monzon fought a top fight beating Valdes at that stage of his career. Hagler tho talented was probably more green than Monzon was declined. His youthful vigor and energy would be offset by Monzon's experience and canniness IMO.
Hagler is my 2nd favorite fighter ever but I believe even in their respective peaks, Monzon takes a very hard fought close decision despite Marv's success against taller fighters.
Marvin wasn’t even ready for a championship fight against Vito at the end of 1979. Monzon schools him at this stage.
Even though this thread has been done over and over, Carlos Monzon by unanimous 15 round decision over Marvelous Marvin Hagler, who I think was a very great champion as well. Experience is the factor here, Monzon's record when he retired was 89-3-9, 61 KO's, undefeated in his last 82 fights, dating back to Oct 9 1964,decked twice in career, Jorge Fernandez, 1966, and Rodrigo Valdes, 1977. Retired with belt after 14 title defenses. Champion from 1970-1977. I think Monzon was at the very end of his career in 1977, confidence and ego were also factors.
Hagler not ready yet at this point. It's unfortunate that although their careers overlapped, they didn't overlap at the right points in their careers.