Roy took a lot of flush shots against Tarver in their first fight and was only ever maybe rocked a little bit, he was not chinny imo. After getting caught with a monster punch in the rematch I suspect it shattered his confidence in his own ability. If you're genuinely terrified of getting iced out it makes it that much more likely to happen, not to mention his reflexes rapidly failed him at the same time. All we eventually learned was that his chin wasn't great, not that it was ever bad.

I think Whittaker is a good example of the same decline, only he had a wicked jaw all along. Completely relied on his reflexes, was so elusive that he'd never had to demonstrate a great chin in his prime. Once he was physically shot we learned that he in fact was tough as all hell to boot.