Arsenal have confirmed that Sead Kolasinac has sustained a "significant shoulder injury" ahead of a meeting with Olympiacos at Emirates Stadium.
Kolasinac was forced off the pitch during the first half of the Gunners' 3-2 home win over Everton on Sunday, which marked his first appearance in six weeks following an ankle problem.
He has now been dealt another setback which is likely to limit his contribution to the team heading into a crucial period of the campaign.
Mikel Arteta admitted post-match that Kolasinac was in a "lot of pain" in the dressing room, but said he was hopeful that the defender had avoided a dislocation.
The club have now released an official statement saying Kolasinac will "undergo further specialist assessments during this week" before he undertakes a rehabilitation programme.
Arteta will now have to make do without the Bosnia and Herzegovina international for a Europa League round-of-32 second-leg clash against Greek opposition on Thursday night, with Bukayo Saka likely to fill in at left-back.
Saka has performed admirably in an unnatural role over the last couple of months having played in midfield in the first half of the season under Unai Emery.
The 18-year-old has racked up 26 appearances for the Gunners across all competitions this season, and Arteta is optimistic that he will be offered an extension on his current contract, which is set to expire in 2021.
Arsenal will be favourites to progress in the Europa League after beating Olympiacos 1-0 at the Georgios Karaiskakis Stadium last week, before focus shifts to an FA Cup tie against Portsmouth on Monday.
The north London club have also confirmed that they expect Kieran Tierney to return to training by the end of February after recovering from a dislocated shoulder, which means he could be available for the trip to Fratton Park.
January loan signing Cedric Soares could also be in line for his debut having been working on his fitness in the gym following a knee issue which he suffered while playing for Southampton.
Arsenal won't be back in Premier League action again until March 7, with West Ham set to arrive at the Emirates for a London-derby clash.
Arteta's men moved to within seven points of fourth-placed Chelsea after beating Everton but must string a few more wins together in order to stay in the hunt for Champions League qualification.