AC Milan have confirmed that Sullery Muntari has undergone surgery on a ruptured anterior cruciate ligament.
The Ghana international joined the club on loan from city rivals Inter in January and he impressed with three goals in 13 appearances.
He agreed a two-year deal with Milan last month, but now looks set to be out of action until 2013.
Milan said: "Muntari underwent surgery for the reconstruction of the anterior cruciate ligament in his left knee.
"The recovery time is around five to six months, excluding complications."