Arsenal have confirmed Kieran Tierney suffered a dislocated right shoulder during their 3-1 win at West Ham on Monday.
The left-back started the game at the London Stadium but was forced off with the injury after just half an hour.
The news comes as another blow for Tierney, who has struggled for fitness since arriving from Celtic in the summer.
A hip injury meant he did not make his debut until late September, and he has made just 11 Arsenal appearances.
Tierney will miss Arsenal's trip to Standard Liege in the Europa League on Thursday, with the club saying he will undergo assessment.
The 22-year-old will be joined on the sidelines by Granit Xhaka, who suffered a concussion against West Ham and will miss the game against Standard and Sunday's game against Manchester City - live on Sky Sports.
Nicolas Pepe and Hector Bellerin will also miss the trip to Belgium - with knee and hamstring injuries respectively - and will be assessed ahead of City's visit to the Emirates.