The defender left Parma for Milan last month.
"I was happy at Parma and I wasn't there against my will, but I was then given a chance that I had never been given before," the 31-year-old said.
"I thought I would end my career there at Parma, but then Milan came along and I couldn't say no.
"I have put myself on the line, I know that here it will be a lot harder to get a game than at Parma. To come here I sacrificed a year of my contract and I have signed until June 2016."