Kaka's injury is reportedly more serious than first thought and he could be forced to undergo surgery over the next few days.
The Brazilian star skipped AC Milan's doomed trip to Juventus on Sunday, officially due to an upper thigh problem.
However, reports in the Italian press on Monday allege the Rossoneri medical staff are now fearing the worst, as Kaka still feels pain in the groin area.
According to La Gazzetta dello Sport, he could be suffering from inguinal hernia, the same problem that has beset defender Daniele Bonera.
Kaka is expected to undergo further tests this week and should this diagnosis be confirmed, the 26-year-old will be operated on as soon as possible in order to reduce his recovery period.
However, he would certainly miss the winter training camp in Dubai, which will start on Dec. 29.
The Diavoli were already dealt a big blow last week, as midfielder Gennaro Gattuso was ruled out for up to six months after damaging knee ligaments.
On Monday morning, the Italian international will leave for Antwerp, Belgium, where specialist Marc Martens will perform the surgery next Friday.