Its bad news to be sure... but I don't think its as big of a deal as everyone seems to think.
This isn't Cannavaro circa 2006 when he was the CB on the planet... this is two years later, and he is not exactly the player he was.
If I'm Donadoni, I'm on the phone with Nesta. If he refuses, Gamberini.
I would arrange my defense like this:
Grosso-Chiellini-Nesta/Gamberini/Materazzi-Zambrotta
That's one mean, mean defense. If Chiellini did that to his own teammate, imagine what he'd do to France, Romania and Holland!
Lets face it... Italy wasn't going to depend on Cannavaro, Nesta, Materazzi, Grosso and Zambrotta come 2010... they may as well get the next group some valued experience.