It would be a toss-up, with a slight edge to Monzon.Hagler may pull off the upset in the right circumstance.
The Hagler that defeated Cyclone Hart in 76 and onwards, defeats any version of Monzon, imo. Superior all round boxer, was Hagler.
I think Hagler might get Monzon in '77, but I ain't betting big bucks on that... Hagler was still green at that stage of the game... However, yes, no doubt Monzon was fading at age 35... STILL! I'll give it to Monzon in '77 by a close decision... BUT! I'll pick '81 Hagler over '71 Monzon in a time machine... MR.BILL
In 1977 you would have to give the edge to the experienced Monzon...I've mentioned this before, but what really makes me wonder about this match-up is how Monzon would deal tactically with a mobile southpaw and how it would affect his own jab output...with Monzon everything comes from that jab...and it is difficult to establish a jab against southpaws. Just food for thought people...
Hagler beat Monzon by majority decision, with one fighter's ability going down, and the other just right to get the job done.