The Italy international suffered the injury in the first half of Saturday's 1-0 Serie A defeat to Roma at the San Siro, and was replaced by Clarence Seedorf.

A statement from AC Milan read: "Pirlo underwent tests on his injury which revealed a muscle strain to his right thigh. The midfielder is set to be sidelined for around four weeks but will join up with the squad for warm-weather training in Dubai."

Pirlo is set to miss the first three league games of 2011 but could return to action when AC Milan host Cesena on January 23.