Arsenal have suffered their second midfield injury blow in a week after Mikel Arteta picked up a calf problem that manager Arsene Wenger said "looks quite serious."
Club captain Arteta, 32, had returned from a short spell on the sidelines to play in the weekend's 2-1 home defeat to Manchester United.
That game saw England international Jack Wilshere sustain an ankle injury that will keep him sidelined until the New Year and may require surgery.
Arteta was substituted midway through the second half of the Gunners' 2-0 Champions League win over Borussia Dortmund at Emirates Stadium on Wendesday.
The victory meant the Londoners reached the knockout phase of the competition for the 15th consecutive year, but Wenger's joy was dampened by news of injuries to Arteta and striker Yaya Sanogo, who scored the opener.
"Mikel Arteta doesn't look good -- it's a calf," the Arsenal boss told reporters after the match. "I think he will be out for a while.
"The injuries are difficult to take on the chin. We go into many games, and we need as many people as possible available.
"The injury situation is a problem -- we lost two players tonight. We lost Sanogo as well on a hamstring problem."
Goalkeeping duo Wojciech Szczesny and David Ospina, defender Mathieu Debuchy, attacking midfielder pair Mesut Ozil and Theo Walcott and striker Danny Welbeck are all currently sidelined for Arsenal.
The Gunners travel to West Bromwich Albion on Saturday before hosting Southampton, who are currently second in the Premier League.