The Milan striker is out for the next six months after damaging knee ligaments and, at the age of 37, it remains to be seen if he can fully recover to continue his playing career.
"Last night I read on the Milan website all the messages that you have sent me," he wrote in an open letter.
"I thank each and every one of you, along with all the writers of the many other messages that couldn't be published simply because there was no room.
"Seeing once again the affection that surrounds me gives me great strength. Thank you from the heart."