Mikel Arteta said "it's not looking good" for Calum Chambers, who had to have surgery on a knee injury on Thursday.
Calum Chambers required surgery on the knee injury he sustained against Chelsea and Mikel Arteta suggested that may push Arsenal into the January transfer market.
Defender Chambers was forced off after an awkward fall in the 23rd minute of Arsenal's 2-1 Premier League loss to the Blues at Emirates Stadium on December 29, reportedly damaging his anterior cruciate ligament.
Arteta was unable to provide a timescale for when the 24-year-old may return to action and hinted bolstering his squad may be more of a priority in January.
The Gunners have been heavily linked with a move for Dayot Upamecano, though RB Leipzig are said to be demanding £80million for the centre-back.
"[Chambers] is in London, having surgery...It's not looking good," said the Arsenal coach.
"We're going to be in the market to see for options that can strengthen the team. That's for sure, that's our obligation. We're going to be working on that.
"Obviously, we had a bad injury with Calum, that's going to change our plans at the back. But the reality at the moment is we're not going to be able to do much.
"I'm more concerned at the moment to get people back from injuries than signings, and trying to improve the players we have here, get everybody on board with what we're trying to do.
"If something extra comes up and we think it's the right opportunity, let's do it."
Arteta claimed his first win at the helm by overseeing a 2-0 victory over Manchester United at Emirates Stadium on Wednesday, but he welcomed a short break in the wake of the festive period.
The Gunners are not in action again until Monday, when they host Leeds United in the FA Cup third round.
"At the moment we're managing a lot of injuries, we haven't been able to rotate the team a lot. We survived. I was a little bit scared [against United], we could expose a few players and get some injuries," said Arteta.
"We went through the game and we suffered but I think all the teams in the Premier League at the moment are in the same position. We went through it, we suffered together and now we have a few days to recover."