United States midfielder Christian Pulisic has a torn muscle in his right calf, AC Milan said on Monday following an MRI, and he could be out until January.
Pulisic was injured during the first half of Milan's 2-1 loss at Atalanta on Friday.
Milan said he would undergo another MRI in a week.
Pulisic has been one of Milan's top players this season with five goals and four assists in Serie A. He has also scored three goals in the Champions League.
The USMNT doesn't play an official match until facing Panama in the Concacaf Nations League semifinals in March.
Pulisic could return for the Italian Super Cup semifinal matches against Juventus in Saudi Arabia on Jan. 3.