We have no idea what kind of stamina Stevenson would have in fights longer than three rounds. What he’d be like training for one specific opponent who is specifically training for him. How he’d react to adversity with a big, strong, seasoned heavyweight pro beating on him for multiple rounds. How he’d react if he got cut. If smaller pro gloves would impact his punch resistance as much as it might enhance his power. But here’s the real thing — and I’ve known some people who have trained some Cuban fighters or others from less fortunate countries — how would he react to freedom? What happens if he goes to camp in, say, America and can decide to half-ass his training (with no government sponsor to threaten his family or living conditions or put him to work in the cane fields if he doesn’t want to work hard at boxing, etc.) or eat like a kid in a candy store? We know he had issues with the bottle (at least in his later days) … would he crawl into one given that freedom? He would have been a fine prospect. Perhaps the best prospect in heavyweight history. But we’ve seen plenty of amateur stars flame out in the pros for a multitude of reasons (and some who have succeeded). We have no idea how he’d turn out. There was a big Cuban pro who beat Riddick Bowe and Lennox Lewis in the amateurs. What became of him?
All else aside I think trained for 12 rounds I think he beats Frazier but runs into trouble with Foreman in a brawl he comes out second best. Now if trained for years as a pro could be different.