Arsenal centre-back Thomas Vermaelen will be out for about a month after having minor surgery on his left ankle.
The 25-year-old suffered the injury in the Champions League tie with Udinese.
"Vermaelen has undergone a minor procedure to remove an inflamed plantaris tendon from his left ankle," read an Arsenal statement.
It is another blow to the Gunners, who have started the season poorly and recently lost Jack Wilshere for up to three months with an ankle problem.
Vermaelen missed most of last season because of an Achilles injury to his other leg, which also required an operation.
"This is the same treatment that he had on his right ankle in January this year and has nothing to do with his Achilles tendon," the statement continued.
"The action was taken after consultation with specialists who advised this was the best thing to do to prevent it becoming a persistent problem."
Vermaelen picked up the problem during Arsenal's 2-1 win at Udinese on 24 August and the club had hoped he would not require surgery.
But he had the operation in Stockholm on Monday and is now set to miss matches against Swansea, Borussia Dortmund, Blackburn, Shrewsbury, Bolton, Olympiakos and Tottenham.
The former Ajax defender was sidelined for the Gunners' 8-2 thrashing at Manchester United on 28 August, but is a first-team regular when fit for Arsenal.
Per Mertesacker, who signed just before the transfer window shut, may be called into immediate action, while Laurent Koscielny, Johan Djourou and Sebastien Squillaci are also centre-back options available to Arsenal boss Arsene Wenger.
The Gunners have conceded 11 goals so far this season and are 17th in the Premier League table with one point from three games.