Pato included in Milan squad
The Brazilian missed Saturday's 0-0 draw at AS Roma but a club statement said he would travel to Manchester, where the Serie A side need to overturn a 3-2 first leg deficit.
Pato is the key part of the three-pronged attack favoured by Milan coach Leonardo, who was forced to play strikers Marco Borriello and Klaas Jan Huntelaar as a front two at Roma.
If the Brazilian is fit enough to start on the right of the trio, Dutchman Huntelaar is likely to drop to the bench with Ronaldinho and Borriello filling the other forward positions.
David Beckham would have been a candidate to replace Pato but the 34-year-old is now more likely to play on the right of midfield if Leonardo decides to bring the England man back against his former club.
Midfielder Mathieu Flamini played instead of substitute Beckham against Roma while Gennaro Gattuso is available at Old Trafford after a ban and Clarence Seedorf returns from a knock.