Kieran Tierney could be sidelined until March after dislocating his shoulder during Arsenal’s 3-1 win at West Ham on Monday night.
The defender suffered the injury during the first half of the victory after an innocuous challenge with Michail Antonio.
Tierney was immediately replaced by Sead Kolasinac - who went on to set up Arsenal’s equaliser - and was taken to hospital for treatment later that evening.
Speaking after the game, interim boss Freddie Ljungberg said: "Tierney, I think it's something with his shoulder but I don't know exactly what it is.
"But obviously we had to take him off so I think he's going to maybe go to hospital and check what it is.”
Arsenal have yet to put an official timescale on his return, but Goal has learned that the Scotland international will be out for up to three months. It has not yet been decided whether he will undergo surgery.
The news comes as a massive blow to Arsenal and to Tierney himself, who has been beset by injury problems following his £25 million ($30m) move from Celtic in the summer.
When he signed he was still recovering from a double hernia operation he had undergone earlier in the summer and had to wait until September 24 to make his debut.
Arsenal have carefully managed his return, with the 22-year-old even being allowed to miss out on the last two Scotland squads so that he could work on his fitness in London.
He has only made 11 appearances in all competitions and is now not expected to add to that until February at the earliest.
Granit Xhaka, meanwhile, will miss Thursday night’s Europa League tie with Standard Liege and Sunday’s meeting with Manchester City in the Premier League.
The Switzerland international suffered a head injury against West Ham and his return will now be managed while following the football’s concussion protocol.
Nicolas Pepe (knee), Hector Bellerin (hamstring) and Shkodran Mustafi (suspended) will also miss the game against Liege. Pepe and Bellerin will then be assessed ahead of the Sunday’s clash with Manchester City.
Rob Holding (knee) is around two weeks away from returning to full training, as is Dani Ceballos (hamstring).