I was about to say, "Naseem Hamed", but changed my mind.

Yes, his last BIG fight was a globally resounding, humiliating loss, and his only and last fight afterwards was an appalling 12 round display of bumbling against some 35 y.o euroscrub who managed to steal a few rounds. You could say his former glory SHOULD have carried him to great heights.

There again, we can all say, "If only he'd stayed with Ingle", "If only Cus D'Amato didn't die so soon", etc. Barrera managed to restyle himself utterly for that fight. Hell, even that bullhead Morales learned to throw a single jab every minute for a short while in '02/'03.

Nas didn't adapt because he couldn't, which is why some bright spark finally caught up to him.