I'm assuming that any person in the history of the world could possibly be knocked out by a hard shot. Hagler had as granite of a chin as there ever was in boxing history. So is it possible he got KO'd? Sure. But it would be about as likely as McGregor beating mayweather in a boxing match.
Not only did hagler have a great chin, but he was a good enough boxer that he didn't just get hit flush over and over. It's not as if he didn't know how to move if he were hurt.
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