Arsenal manager Arsene Wenger could head into the busy program of fixtures over Christmas without any recognized full backs after Andre Santos injured his right ankle in the Champions League loss to Olympiakos.
The Brazilian left back, one of the few first-team regulars selected for the 3-1 defeat on Tuesday, was forced off early in the second half.
Wenger says "Santos suffered a bad ankle injury. He looks like he'll be out for a while."
Left back Kieran Gibbs has been out since last month because of a hernia and may only be fit to return in three weeks. Right backs Carl Jenkinson and Bacary Sagna are also injured.
From Saturday, Arsenal plays six Premier League matches in a 24-day span.