Ospina suffered a knock to his back in a seemingly innocuous collision at the weekend in the 2-1 win over Crystal Palace, meaning Szczesny, who featured just twice in eight games since being dropped in early January, could be restored to the starting line-up.
If Ospina is sidelined through injury at the Emirates on Wednesday, he will join Aaron Ramsey (hamstring), Alex Oxlade-Chamberlain (groin), Mikel Arteta (ankle) and Mathieu Debuchy (shoulder).
Monaco may have both Belgian winger Yannick Carrasco (thigh) and France defender Layvin Kurzawa (bruised quadriceps) missing through injury.
The pair face a race against time to be fit for the first leg, with teenager Almamy Traore set to make his first ever competitive start if Kurzawa is ruled out.
The Ligue 1 side will definitely be without Joao Moutinho, who is suspended, while Ricardo Carvalho, Tiemoue Bakayoko and Andrea Raggi remain injured.