The Brazil international has spent the past few weeks on the sidelines due to muscular problems, and recently travelled to the USA to determine the cause of his ongoing injury worries.
Pato returned to Italy on Saturday though, and has now been deemed fit enough to feature in Tuesday's encounter in Catalunya.
Milan will be looking to book their ticket for the semi-finals after recording a scoreless draw in the first leg at San Siro. Tuesday's match kicks off at 20.45CET.
Goalkeepers: Abbiati, Amelia, Piscitelli
Defenders: Abate, Antonini, Bonera, De Sciglio, Mesbah, Mexes, Nesta, Zambrotta, Yepes
Midfielders: Ambrosini, Aquilani, Boateng, Emanuelson, Nocerino, Seedorf
Attackers: El Shaarawy, Ibrahimovic, Maxi Lopez, Pato, Robinho