Sead Kolasinac is facing several weeks on the sidelines after Arsenal confirmed he suffered ankle ligament damage against Manchester City.
Kolasinac hobbled off in the first half of his side's 3-0 defeat to City last weekend.
In an injury update on their official website on Friday, Arsenal revealed the 26-year-old defender is "aiming to return to full training in January".
Arsenal are already without fellow left-back Kieran Tierney until March after the Scotland international underwent surgery on his dislocated shoulder earlier this week.
And in a further blow for the Gunners ahead of Saturday's Premier League meeting with Everton, Hector Bellerin, Rob Holding and Mesut Ozil are all fitness doubts.
Bellerin (hamstring) and Holding (knee) played no part in the defeat to City, while Ozil injured his right foot prior to being withdrawn in the second half.
Dani Ceballos is aiming to return to training next week after missing Arsenal's last six Premier League games, while Granit Xhaka is available for selection after recovering from the concussion he sustained against West Ham.
Arsenal are 10th in the Premier League ahead of their trip to Goodison Park, seven points adrift of the top four.