Cassano caused shock when he moved across San Siro in exchange for Giampaolo Pazzini plus 7m.
"I didn't like that he decided to speak out after he left, nor do I agree with what he said," said Bonera. "That's just what he's like. On the field he gave us a big hand and the 2011 Scudetto also belongs to him.
"The club stood by him when he was ill, and that's not to be sniffed at."