Pato sidelined for six weeks
Pato suffered the latest in a series of problems in Milan's 3-1 win over Palermo on Wednesday.
"A recovery time of around six weeks is estimated," a Milan statement said. His injury comes as a fresh blow to the Serie A leaders, who already lost fellow striker Filippo Inzaghi for the season after the 37-year-old suffered knee ligament damage on Wednesday.
Zlatan Ibrahimovic is now Milan's only out-and-out frontman with Robinho, Ronaldinho and Clarence Seedorf offering options as a second striker.
Pato, 21, is top scorer for the new Brazil side coached by Mano Menezes with a goal in each of the team's three friendly wins against the United States, Iran and Ukraine since August.
The Brazilian federation (CBF) said a replacement would not be called up for the friendly next Wednesday.
Menezes still has Andre, Neymar, Robinho and the recalled Ronaldinho as forwards.