Rob Holding could be sidelined for nine months after rupturing an anterior cruciate ligament against Manchester United on Wednesday.
Arsenal, who face Huddersfield at the Emirates this afternoon, announced yesterday that defender Holding, 23, had severely damaged his left knee and would be out for between six and nine months.
Midfielder Aaron Ramsey (ankle) is a doubt for the Terriers' clash and Mesut Ozil (back) is unlikely to feature once again, but Granit Xhaka is available following suspension.
Defender Laurent Koscielny (Achilles) is nearing a return, with Thursday's Europa League tie with Qarabag most likely, while Nacho Monreal (hamstring), Konstantinos Mavropanos (groin) and Danny Welbeck (ankle) remain sidelined.
Meanwhile, Arsenal say they will "remind players of their responsibilities" after a video emerged showing some Gunners stars inhaling nitrous oxide.
Players including Ozil, Pierre-Emerick Aubameyang, Alexandre Lacazette and Matteo Guendouzi feature in the nightclub CCTV footage which was published in The Sun.
Arsenal said: "The players will be reminded of their responsibilities as representatives of the club." Nitrous oxide, also known as laughing gas or 'hippy crack', is freely available but it is technically illegal to sell it for recreational purposes.
In the video, reportedly from a London venue shortly before the start of the season, other Gunners players including Henrik Mkhitaryan can be seen rejecting the offer to use the gas.
This is the latest incident of top players being linked with nitrous oxide.
Raheem Sterling was warned by Liverpool after footage emerged of him using the substance in 2015, while Jack Grealish was warned by Aston Villa and Saido Berahino was reprimanded by West Brom in 2014.