Bonucci left Juve last summer for the Rossonero.
“It was a difficult choice, moving to Milan, because I was accustomed to a certain standard and that includes the top tier of European football," said the former Juventus man.
“Taking a step back was tough, but Milan convinced me straight away with their project and enthusiasm, which we are now rediscovering.
“Now we have the opportunity of going all the way in the Europa League and Coppa Italia. I am convinced we can have our say and keep that hunger for success alive.
“It costs nothing to dream and we've got what it takes to reach the various Finals."