Arsenal manager Arsene Wenger says both Abou Diaby and Wojciech Szczesny are out for three weeks with respective thigh and ankle injuries.
Diaby was replaced after 17 minutes during Saturday's defeat to Chelsea, while Szczesny, who injured himself in the warm-up against Southampton, has missed The Gunners' last two league games.
Both players will miss Arsenal's Champions League game against Olympiakos, while Mikel Arteta is a doubt for the game with an ankle problem. Aaron Ramsey is likely to deputise.
But Wenger did have better news on Jack Wilshere, who came through his return from 14 months out injured against West Bromwich Albion for the Arsenal Under-21's unscathed.
"It's like all players when they come back for their first game: they realise how difficult and tough it is," Wenger said.
"You always have to make that first step, and he has done that and that is fantastic.
"He is a quality player, it is great for the club that he has taken that first step on the road towards getting back into the first team. I think he is delighted to have been back on the pitch, as we all are.
"It is a start, now he just needs to keep working hard to play the football we know he is capable of playing, which we know is very exciting. When he runs with the ball, it is very close to him and as he runs he always has his head up and is looking for opportunities to play people in.
"What you can't duplicate in training is that match sharpness and players coming in at you, so he will have benefited from that in this match."
Wilshere's return has coincided with an injury crisis in midfield for Arsenal, with both Arteta and Diaby out injured, but Wenger is unlikely to be pushed into bringing back the England international given his sustained period on the side-lines.
Ramsey will deputise for the game against Olympiakos at the Emirates Stadium, where Wenger will be hoping his side can continue their winning start to the competition after they defeat Montpellier on matchday one.