Hamstring rules Pato out
The Brazilian limped off on the 20-minute mark and was replaced by Stephan El Shaarawy, who scored Milan's equaliser on his San Siro debut to earn them a 1-1 draw.
A statement on the club's website read: "AC Milan communicates that the tests which Alexandre Pato underwent this morning showed damage to the biceps muscle of the right thigh. The prognosis is for a recovery period of about four weeks unless there are complications."
Pato's latest problem adds to an astonishing injury list at Milan, which also includes fellow attackers Zlatan Ibrahimovic, Robinho and Kevin-Prince Boateng, midfielders Gennaro Gattuso, Massimo Ambrosini and Mathieu Flamini, and defenders Luca Antonini, Philippe Mexes and Daniele Bonera.