Kaka, Milan's top scorer with 11 Serie A goals this season, netted a penalty in the match and finished the game but immediately afterwards it became clear that he had a problem.
Milan issued a statement on Sunday saying that tests showed he has sprained rather than broken his foot but that he faces at least two weeks on the sidelines.
The news could not have come at a worse time with the rossonero's crucial clash against neighbours Inter Milan at the San Siro next weekend.
Milan dropped to eight points behind the champions and leaders following their draw as Inter won 3-0 at Lecce on Saturday.
Anything other than a second victory over Inter this season -- Ronaldinho scored the winner with a rare header in a 1-0 success in September -- could see Milan's title hopes all but disappear.