The Brazil international has suffered yet another setback in his quest for full fitness as he picked up a knock in Tuesday's quarter-final loss in Europe's elite competition
The 22-year-old attacker made his comeback after a recuperation in the USA in the second leg of the quarter-final against the Catalans, but limped off the pitch only 14 minutes after replacing Kevin-Prince Boateng.
It's not yet exactly known how long Pato will be unavailable due to the thigh injury, but AC Milan coach Massimiliano Allegri fears that the forward could be out of action for a while.
"He felt pain in his thigh again. The medical staff will have a look at it. It's a serious problem if he's picked up yet another injury. He's been struggling for two years now," Allegri said to reporters.
Pato has made only 16 appearances in all competitions this season due to a string of injuries.