Arsenal have confirmed that left-back Andre Santos is set to undergo surgery in Brazil on his ankle, ruling him out for three months.
The Brazil international picked up the injury in the Gunners' Champions League group-stage defeat against Olympiakos last Tuesday, with the English Premier League club having already qualified for the knockout stages.
There had been hopes that the injury was not too serious, with the former Fenerbahce defender missing out on Saturday's 1-0 victory over Everton at the Emirates.
But Arsenal have since revealed the 28-year-old will be sidelined until next year.
"Following investigations and subsequent medical opinions by club specialists, Andre Santos will have surgery to repair ligament damange in his right ankle," a statement on the club's website read.
It was thought Santos would only miss three weeks, but he now faces a lengthy spell on the sidelines alongside fellow full-backs Bacary Sagna and Kieran Gibbs.