"He sustained up a pretty serious sprain. We need 48 hours to evaluate the extent of the injury," said Milan coach Carlo Ancelotti.
It remains to be seen how long he will be out for, but the Brazilian is a real risk for the first leg of the Champions League clash with Arsenal on February 20.
Ancelotti added: "We are back in an emergency situation. With Gilardino suspended, we hope that Pippo Inzaghi or Ronaldo can recover for next Sunday.
"Pato's goal was very well constructed. Kaka did a great job in attacking the space left by Gilardino and putting it straight into the centre for Pato, who had a marvellous control and finish."