How Does a Fighter Lose His Chin?

Welcome to The Random Boxing Blog. 

I was bored last night so I watched Pacquaio vs. Hatton again. What a dramatic fight that was. 

The funny thing is: people in the post-fight analysis kept referring to how Hatton can no longer absorb a punch. Whilst it's true that in the Tsyzu fight, Hatton did walk through some bombs, the idea that a fighter's jaw can suddenly desert him is a strange one. Normally a fighter's chin is something referred to us a natural born gift; something you can't train or "put muscles on". 

So how is it that it can suddenly become more china-like? Or put another way: could Ricky Hatton in 2005 really take a better shot than Ricky Hatton in 2009?