Cannavaro was horrible for half the season, clearly the Real Canna we all feared. Legro? Please. Chiellini? Maybe. Marchisio... well, I'm not surprised by that. You seem to rate him higher than anybody on this forum.
But who wasn't horrible for (almost) half the season?
Do you remember away to Napoli, home to Palermo, away to Chievo, home to Milan, away to Bari, home to Fiorentina, away to Palermo. Those are all games were Diego was, for me at least, absolutely terrible. Not just bad or mediocre. And that's not even talking about how he performed in the CL.
And, in case you don't understand it, I don't even think Canna, Marchisio, Buffon or Legro had a good season last year, because they didn't. But I do think they were "better" than Diego.
The only player who had a good season (mind you, not excellent) was Chiellini. Unlike this season, sadly.
Edit: seems like a lot of people, for whatever reason, think that when you say player A performed better than player B, that automatically means that player A must have been good. Odd.