The former Portsmouth midfielder is facing a month on the sidelines with a thigh problem sustained in Sunday's Derby della Madonnina defeat by Inter at San Siro
The Rossoneri released a statement on Monday evening revealing that the versatile midfielder had suffered a thigh injury in Sunday's Serie A clash with Inter and will be sidelined for a month "unless there are complications".
Milan also gave an update on the fitness of goalkeeper Christian Abbiati, who was in obvious discomfort during the Derby della Madonnina.
"[He] is suffering from a muscle strain in his left calf and will be evaluated over the next seven days," the statement read.
The loss of Boateng is a major blow for Milan given that fellow midfielders Gennaro Gattuso and Alberto Aquilani are also out of action at the moment, but the Italian champions have at least signed Genoa's Alexander Merkel on loan until the end of the season.