The 23-year-old, who has been plagued by injury problems for much of his career, suffered a groin muscle tear just four days before the start of the current season.
He will almost certainly not be in the squad for Sunday's derby against local rivals Internazionale. However, if his return to training goes as well as is hoped he may be involved when the Rossoneri travel to face Lazio after the international break.
The Brazil international has not played for Milan at all since the Champions League tie at Barcelona back in April, although he recovered from that injury in time to appear for Brazil at the London Olympics.
With Pato injured and Zlatan Ibrahimovic sold to Paris Saint-Germain, Giampaolo Pazzini and Stephan El Shaarawy have had to take care of the goal-scoring duties. They are the only two players to score for the club in Serie A this season. El Shaarawy has scored for of Milan's seven league goals thus far.