Hagler would've only had trouble with Monzon who retired just months before a fight could be arranged. It is probably fortunate for Hagler that Monzon did retire as his first attempt at winning the World Middleweight title ended in a draw in early1979.

I found this interview with Hagler talking about past Middleweights...
HAGLER: Well you know those are great champions and because I think I was a throwback from the old days, like a Sugar Ray Robinson and Gene Fullmer and guys I would say like Jake Lamotta, it was something that I had to do to bring back the respect to the middleweight division. At the time coming up, when the heavyweight division was really notable by Muhammad Ali and Joe Frazier and all these tough guys in the light heavyweight division, you had Bob Foster. In the middleweights we were lacking something, so I was very honored in the sense that I finally started gaining my recognition as one of the top middleweights in the world. But for me still, I feel as though Carlos Monzon was a great champion. It took ten years and we would have loved to have fought him if we had the opportunity, but unfortunately that never happened. So just continuing and looking at guys like Emile Griffith and all these great middleweights. It’s always been the toughest division out there in the world of boxing I feel.

Asked what approach he would take to Monzon?

HAGLER: Well I believe behind my orthodox style I’d probably be fighting him on the inside to get inside his long reach. Just knowing that this guy had a powerful right hand, I mean Monzon had one of the best right hands in the business also besides Tommy Hearns. I couldn't switch against Monzon, his right hand would be a danger, i had learned a lot and i think i could've given him fits. He was tough too and had an iron chin.

Trinidad would have been in trouble with Hagler in every way. I only give Monzon any chance of beating prime Hagler.