Arsenal boss Arsene Wenger has confirmed Francis Coquelin will be out for a minimum of two months with a knee injury.
Coquelin had to be substituted during the first half of Arsenal's 2-1 defeat at West Bromwich Albion on Saturday, and there have been suggestions he has suffered ligament damage.
"I'm always a bit cautious," he said. "It is at least two months but I'm cautious as we had so many bad surprises on scans that I do not want to speculate more than that. Certainly for the next two months we play without Coquelin."
Arsenal host Dinamo Zagreb in the Champions League on Tuesday and Wenger said Mikel Arteta will also be out with a short-term calf problem.
The Gunners boss added: "I must say we have been hit very hard in November. We have lost a few players."
However, Wenger confirmed Aaron Ramsey, who has returned to training after a hamstring problem, will be available for the Zagreb game, although he may not be risked from the start.
"I have not completely decided," Wenger said. "I have 24 more hours."
While the January transfer window will give Wenger the chance to bring in another midfield recruit, he suggested he already has a number of options available to step into Coquelin's place.
"It is of course a disappointment to lose him in the longer term but we have in the squad the players who can compensate," he added. "We have lost a player of quality for a while but we know that can happen
"I will give myself time, we have players who play in this position like Mathieu Flamini and players who can play there like Calum Chambers.
"Sometimes this is a good opportunity for other players to turn up and do the job. I will do what is needed in January, knowing as well it is not an ideal transfer market."
Coquelin has been at Arsenal since 2008 but was on the fringes of the team until being recalled from a loan spell at Charlton in December. Since then he has established himself in the team but his absence will again draw attention to the fact Wenger did not bring in any outfield reinforcements over the summer.