Bellerin is back in action after overcoming a knee injury and makes his first start since taking part in the 3-0 loss to Manchester City on March 1.
Monreal returns from a back problem, while fellow defender Shkodran Mustafi is also in the XI after being passed fit to play despite having limped off with a groin problem in Sunday's 3-0 win over Watford.
For Milan, defender Davide Calabria has not travelled with the squad, which means that Gennaro Gattuso has picked former Liverpool forward Fabio Borini to feature at right-back, having started in that position in their 1-0 win over Genoa on Sunday.
Arsenal: Ospina; Bellerin, Mustafi, Koscielny, Monreal; Xhaka, Ramsey; Wilshere, Ozil, Mkhitaryan; Welbeck
Subs: Cech, Chambers, Kolasinac, Elneny, Maitland-Niles, Iwobi, Nketiah
AC Milan: Donnarumma; Bonucci, Romagnoli, Rodriguez, Suso, Kessie, Montolivo, Calhanoglu, Borini, Cutrone, Silva
Subs: Storari, Musacchio, Zapata, Biglia, Bonaventura, Locatelli, Kalinic