Chelsea's on-loan midfielder Marco van Ginkel is expected to be sidelined for up to two months with a right ankle injury.
The 22-year-old got hurt last week following a challenge from teammate Sulley Muntari in training and tests done on the Dutchman revealed a right-ankle sprain with capsular-ligament damage.
Milan have not confirmed the estimated recovery time but the Dutchman is expected to be out of action until February.
"I am really sorry for Van Ginkel,'' AC Milan club administrator Adriano Galliani told the club's official website. "What a pity. After 30 years of watching training sessions, I know that he is a player with great talent.
"We hope he will recover as soon as possible and that he is able to showcase his talent. He has extraordinary feet, he is a good person and intelligent.
"All my best wishes for him for a speedy recover in order to give a helping hand to our team.''
AC Milan coach Filippo Inzaghi was also disappointed to lose Van Ginkel.
"It's a shame,'' he told acmilan.com. "It was a hard challenge but not premeditated. The first one to apologise was Sulley.
"If we didn't train as intensely as we do then we wouldn't win games. It wasn't a killer tackle, otherwise I would have intervened.
"We are disappointed because we lose a player that would have played and who was going through a good spell. Our thoughts are with Marco, it's sad to see him leave with crutches.''
Van Ginkel has made two Serie A appearances for Milan since joining the Italian outfit on a season-long loan from Chelsea this summer.