Kieran Tierney has posted an update on social media after successfully undergoing an operation on his dislocated shoulder.
The former Celtic left-back fell awkwardly and had to be withdrawn on the half hour mark during the 3-1 victory at West Ham last Monday.
Speaking after the game, interim head coach 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."
Further diagnosis revealed the 22-year-old damaged his shoulder, and a spell of up to three months on the sidelines is expected for the Scottish international. Tierney is to continue receiving consultations on his shoulder, with an expected return date not yet clear.
The injury is a major setback for Tierney, who is still to fully establish himself at the north London club since his summer move from Celtic.
The defender had to wait til late September to make his debut due to a niggling groin problem, before suffering a further setback with his hip.
But posting on his Instagram story on Monday, Tierney uploaded a picture of himself in hospital after his operation with the caption: 'Op done, soup terrible, thank u as always for the lovely messages. Helping me a lot,' with a heart emoji.
Tierney's injury is unfortunate timing for Arsenal after Sead Kolasinac was forced off in the first half of the 3-0 defeat to Manchester City on Sunday.
The Bosnian was replaced by Bukayo Saka, who had to fill in as a makeshift left-back against the Premier League champions.
Kolasinac's injury may not be too serious, however, with the 26-year-old able to walk off the pitch unaided.
Arsenal travel to Everton on Saturday, with the Gunners confident of appointing Mikel Arteta as their new head coach before the trip to Merseyside.
The former Arsenal midfielder held talks with Gunners managing director Vinai Venkatesham and club lawyer Huss Fahmy at his Manchester home during the early hours of Monday morning, and the north London side are optimistic of striking an agreement in the next few days.
