Pato scored Milan's winner on Sunday against Fiorentina but was later stretchered off after sustaining a sprain to his left ankle.

The South American youngster, widely regarded as one of the most promising players in world football, will undergo further tests on Tuesday to confirm the extent of the injury.

However, Ancelotti fears Pato, who has pulled out of the Brazil squad for Wednesday's international friendly against the Republic of Ireland, could be sidelined for some time.

"Pato has sustained a sprain in his left ankle and it is something serious," said Ancelotti. "We will have to wait 48 hours to find out more.

"It's a pity because he has scored a great goal, but now we have a crisis in attack, with Alberto Gilardino and Pato unavailable for next Sunday's game. We hope Ronaldo and Filippo Inzaghi recover from injury in time."

Sunday's result helped Milan climb to fifth in the standings, four points behind Fiorentina, who hold the last Champions League qualifying spot.

The Rossoneri will be desperately hoping Pato's injury is less serious than feared and that the player will be available for the Champions League clash against Arsenal on February 20.