Brazilian ace suffers thigh injury.
The 20-year-old netted twice as the Rossoneri swept aside Bortolo Mutti's men at San Siro on Sunday, but pulled up sharply towards the end of the game, and manager Leonardo says the club is now awaiting confirmation on the severity of the injury.
"We will see in the next few days," he told UEFA.com.
"From the way he stopped we thought it was something very serious, but he had just recovered from a similar injury and maybe he was just worried and scared."
Milan travel to Old Trafford on March 10 looking to recover a 3-2 deficit from the first-leg, and will be desperate to have their top scoring Brazilian available for selection.
The former Internacional man has now scored 14 times in 25 games this season. But despite his obvious concern, Leonardo is confident that his squad is strong enough to cope with the absence of his star man.
"I don’t want to focus too much on the importance of Pato but he had a spectacular game today," he said.
"Nevertheless, I think we have the alternatives and should we be forced to do without him because of injury I am sure we have the resources to fill his absence."
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