The Italian champion has announced that the Brazilian forward will be unavailable for up to a month after he suffered a muscle strain in the Coppa Italia win over Novara.
The forward, who came close to joining Paris Saint-Germain last week before committing his future to the Serie A champion, picked up the muscle strain during the club's 2-1 extra time victory against Novara in the Coppa Italia round of 16 on Wednesday.
Pato scored the winning goal in the game, yet Milan's medical staff has examined the Brazilian and stated the player could be unavailable for the next month.
"AC Milan can announce today that [Alexandre] Pato has been subjected to tests and after specialist medical advise, he was diagnosed with an injury the biceps femoris of the left thigh. The prognosis is [he will be out for] three to four weeks, unless there are complications," the club confirmed on its website.
Milan meets Novara again at the weekend as the club looks to bounce back in Serie A after last week's derby defeat to Inter.