Bellerin has been out of action since he ruptured his cruciate ligament in January, while Tierney is yet to feature for the Gunners since he joined from Celtic in the summer with a groin injury.
Arsenal have conceded eight goals in four Premier League matches this season, most recently when they threw away a two-goal lead to draw 2-2 with Watford.
Goal's Charles Watts believes the Gunners need more than just Bellerin and Tierney available:
Charles Watts @charles_watts
Individual errors from the centre-backs obviously led to both goals, but the fact 31 shots were conceded shows how inept the midfield was of offering any protection. Arsenal have conceded 96 shots this season, the most of any side in Europe’s top five leagues. Staggeringly bad.
The Athletic's Paul Conway echoed a similar thought on Thursday despite Arsenal keeping a clean sheet at Eintracht Frankfurt in the UEFA Europa League:
Ainsley Maitland-Niles has been filling in for Bellerin as a makeshift right-back, but he's struggled defensively at times because he's come through Arsenal's academy playing in midfield or on the wing.
On the other flank, Sead Kolasinac has been playing at left-back, but he has not been particularly convincing defensively since he joined Arsenal in 2017.
Bellerin and Tierney will be significant upgrades in those departments, and the former's return will allow Maitland-Niles to contribute in areas in which he's more comfortable.
Their return won't make David Luiz a more reliable centre-back, though, or increase the protection the defence receives from midfield.
While the pair continue their recovery, Emery must make strides to improve in those areas.