Leonard is a smart man, if He could of beaten Hagler in 82 then He would have, but He could not so He did not try.
I don't know if Hagler stops Leonard. Leonard was durable. He survived Hearns' thunder. Duran dominated him but didn't stop him. Leonard would move against Hagler. But I don't see how Leonard beats Hagler. He didn't beat an over-the-hill Hagler in 1987, how does he beat a prime Hagler in 1982? Hagler was too sharp and too versatile in 1982. Leonard would have his moments, but he would lose in a lopside fight.
In 82 Marvin was a master brawler and a master boxer, with great timing. Hagler had a much longer reach and a better jab than Leonard, and Marvin's jab was better and more accurate than Ray's. Leonards superior speed and athletic ability may be a problem for Marvin, but Marvin's accuracy and timing could nullify Sugar's speed. I would go with Marvin by decision in 12 amd maybe by stoppage in a 15 round fight.
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I also choose Hagler,but by very close decision. It would be controversial,though. A reverse image of their 1987 fight.
Leonard was a natural WW at the time, would probably be difficult for him to be effective at 10-15 lbs heavier. On the other hand I believe that Leonard was further from his prime when they met in 1987. Hard to say really. I'd would be very interesting to see if Leonard had kept active with at least 2-3 fights a year (i e no detached retina and no personal issues) and met Hagler in 1984 or 1985 instead.
There's probably quite a lot of truth to this. But if Duran had moved straight up from LW in 1977 and met as a good a welter as Leonard was in 1980 I think it's doubtful he would have won as well. He probably needed those three years to grow more naturally into a WW.