Arsenal have confirmed Kieran Tierney dislocated his shoulder in the 3-1 win over West Ham.
The Gunners left-back is currently undergoing further assessments to determine how long he will be on the sideline.
Arsenal's other full-back Hector Bellerin was injured during the warm-up at London Stadium, and will miss the Gunners' Europa League clash with Standard Liege.
Bellerin will then be assessed ahead of the Manchester City match on Sunday.
Also missing for Arsenal's trip to Belgium on Thursday is Granit Xhaka, who suffered a concussion against West Ham.
The Swizterland international will also miss the Premier League clash with the Citizens.
Nicolas Pepe has also been ruled out of the Europa League tie after the forward bruised his knee against West Ham. He is expected to return for the City match.
Pepe was Arsenal's hero in their comeback victory over West Ham on Monday.
The summer signing produced and assist and netted a goal as Arsenal won their first match in 10.
Interim boss Freddie Ljungberg said of Pepe: "People always ask me about Pepe, and he needs to adapt and that's not so easy, especially with his price.
"But he showed his quality tonight and he can sleep happy. He will get better and better.
"I hope the win changes the mood. They look happy. But we are demanded to win football matches and "I can sense the pressure that we have been under.
"That lifted a bit today. You always hope one win can transform a season - but we can't get carried away."
