According to LaGazzettadelloSport only Theo should be fully fit (Tonali and Rebic will start on the bench) so I guess:
Donnarumma
Calabria - Tomori/Romagnoli - Kjaer - Theo Hernandez
Meitè - Kessiè
Castillejo Brahim Diaz Saelemakers
Rafael Leao
Our "back 5" is the best possible but the absences of Bennacer (DLP), Calhanoglu (Trequartista), Rebic (our best winger), Ibrahimovic and Mandzukic will hurt us big time that's why I expect a very defensive game from Milan (maybe taking advantage of Leao and Diaz pace to create some dangerous counter-attacks)
Everyone but Ibrahimovic will be available for the second leg though