Manchester City have agreed to let the player leave to join Italian outfit AC Milan.
"We are all so sad because Mario was an important player for us," said Mancini. "With Mario we won the Premier League and the FA Cup in two years.
"He's a fantastic player but, for him, this could be an important chance to go back to Italy and play for a big club in Milan. It will be a good chance for him to stay with his family, to play for Milan. I think he can improve and I'm happy he will become one of the best players in the world."