Pirlo sidelined for another six weeks
The Italy international had tests after being hurt in training on Friday and the Serie A leaders said on their website that "the recovery time is estimated at six weeks".
Pirlo, whose contract expires at the end of the season and who has yet to be offered new terms by the Italian club after failing to shine for several months, had only just returned to training after a month out with a thigh problem.
An injury crisis in midfield prompted Milan to sign Dutchmen Mark van Bommel and Urby Emanuelson in January and although both are ineligible for the Champions League, they could now be key to the club's faltering attempt for a first scudetto since 2004.
There was better news for Milan, who have drawn their last two league games, when fit-again centre backs Alessandro Nesta and Nicola Legrottaglie were included in the squad for Saturday's match at home to Parma.