The Rossoneri striker has picked up another knock and could be out for up to three weeks, and will most likely not feature in his team's Serie A opener this weekend
The Brazilian picked up a knock on his left abductor during a training session on Wednesday, which coincided with the arrival of Giampaolo Pazzini in a swap deal involving Antonio Cassano.
The 22-year-old will undergo a scan in the coming days to determined the extent of the damage.
"AC Milan announces that during a game held in today's workout, Alexandre Pato has reported a problem with his left adductor," the team said in a statement. "In the coming days further medical examinations will be made to determine the extent of the damage."
Pato was restricted to just 11 Serie A appearances last season, due to persistent injury problems.