Senate defenseman Kieran Tierney is scheduled for surgery this week for a knee injury that should rule him out for the rest of the season.
Tierney has been struggling with the issue for several weeks, but had previously been able to continue playing it.
After returning from international duty and feeling unwell in training last Thursday, the left-back underwent a CT scan which revealed the extent of the damage.
Tierney duly missed Monday night’s trip to Crystal Palace and went to see a specialist on Tuesday.
It has been decided that the best course of action is for the Scotland international to have surgery and he is set to miss the rest of the season.
Arsenal and Tierney will not know how long he will be sidelined until the operation is completed and the club can start monitoring his recovery.
Tierney’s absence is a blow to Arsenal, who lost 3-0 at Crystal Palace on Monday night without him.
Nuno Tavares came on as left-back but the youngster was taken off at half-time by Mikel Arteta and Granit Xhaka came on in defence.
Arsenal’s injuries were compounded by Thomas Partey who was limping on Monday night and he is a doubt for Saturday’s game against Brighton with a thigh problem.
“That’s what we have, those are the numbers we have,” Arteta said. “We lost Kieran and we lost Thomas now and that’s it.
“With the players we have we will keep trying as we have been doing since the start of the season, but we have never had a great team so we cannot make excuses.
“Today you have to look at yourself and I’m the first because, I repeat myself, it wasn’t good enough.”