The Brazilian, one of only two starters from the weekend win over Wigan Athletic, was forced off in the second half of the Champions League encounter with the Greek outfit.
However, with Kieran Gibbs still recovering from a stomach problem, manager Arsene Wenger admits he is suffering a selection headache for the left-back position.
"Santos suffered a bad ankle injury. He looks like he will be out for a while," admitted Wenger.
"We have Kieran Gibbs coming back in two to three weeks. Hopefully Santos will not be too long but, for Saturday against Everton,
he looks to be short."
The 28-year-old joined the Gunners from Fenerbache over the summer and has made 13 appearances in all competitions following his move, scoring twice.
By Ben McAleer
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