Arsenal midfielder Jack Wilshere has been ruled out for around three months after successfully undergoing surgery on his left ankle, the Gunners have announced.
The England international was substituted during the second half of Saturday's defeat to Manchester United after damaging ankle ligaments and was subsequently sent to a specialist to be assessed.
Wilshere has now gone under the knife and is likely to be out of action until February.
An Arsenal statement read: "Jack Wilshere has had successful surgery on his left ankle and will be out for approximately three months.
"An assessment with a specialist confirmed the need for an operation and the 22-year-old had successful surgery to his ankle in London on Thursday afternoon."
Wilshere also confirmed the operation went well, thanking renowned consultant trauma and orthopaedic surgeon James Calder, while also expressing a message of positivity on his Instagram account.
He also said his situation was put into perspective by the death of Australia cricketer Phillip Hughes on Thursday, two days after he was struck on the neck by ball while batting.
Wilshere wrote: "Ankle needed surgery in the end, went well thanks to the main man James Calder! Work hard for what you want because it won't come to you without a fight.
"You have to be strong and courageous and know that you can do anything you put your mind to. If somebody puts you down or criticizes you, just keep on believing in yourself and turn it into something positive."
He added: "Puts life in perspective when you things like this happen, and makes my injury look like nothing! Thoughts are with Philip Hughes family. Horrible news! RIP."
Wilshere was replaced by Santi Cazorla in the 55th minute of Saturday's game at the Emirates Stadium after sustaining the injury following a sliding challenge by United defender Paddy McNair.
The England man sat out Wednesday's Champions League win over Borussia Dortmund, which saw Arsenal secure their place in the last 16, and he could now miss more than a dozen further matches, including games against Tottenham, Manchester City and Liverpool.
This is the latest injury setback for Wilshere, who has struggled with various problems before and at one point was absent from the Arsenal first team for around 17 months after suffering a stress fracture in his ankle in 2011.
Wilshere will now only be a spectator for Saturday's lunchtime kick-off against West Brom, where Arsenal defender Per Mertesacker has called on his side to show their Barclays Premier League challenge is very much "alive".
The Gunners produced a much-improved, composed display to beat Group D leaders Dortmund 2-0, reaching the knockout stage with a match to spare.
However, with Arsenal eighth domestically following the loss to United, Mertesacker knows they still have plenty of work to do.
Mertesacker said: ''We were very convincing (against Dortmund). Everyone had to learn.
''We have to show character and that is what makes me really happy for the next few games, because we have to sharpen up in the Premier League as well. Saturday is a good chance to show to everyone that we are still alive.
''Champions League football (in the knockout stages) we have secured and that gives us a life, but that has nothing to do with the Premier League.
''Dortmund are a good example of that, because they are already through, but they have suffered a lot in the league. We have to come back in the Premier League and that is the next target.''
Arsenal will need results to go their way on matchday six to overhaul Dortmund and secure seeding for the knockout stage draw - with the likes of Bayern Munich, Real Madrid and Paris St Germain, just ahead of Barcelona in their group, potential opponents for runners-up.
Whatever transpires by the time Arsenal's next European fixtures come around in February, manager Arsene Wenger will be hoping to see some key men back from injury.
Aside from Wilshere, captain Mikel Arteta is being assessed after limping off on Wednesday night with a calf problem.
Goalkeeper Wojciech Szczesny was sidelined by a hip problem, but should return at The Hawthorns, with striker Danny Welbeck rested because of a minor knee injury as was Theo Walcott (groin).
France forward Olivier Giroud - who scored on his return from a broken leg against United - was ineligible after not being included in Arsenal's original European squad, so will be pressing for a start this weekend, while defender Laurent Koscielny was on the bench against Dortmund after recovering from an Achilles problem.
Yaya Sanogo was substituted late on after pulling up with a tight hamstring, having earlier fired home his first senior Arsenal goal inside just two minutes.