Arsenal are likely to be without defensive midfielder Francis Coquelin for Wednesday's north London derby.
Coquelin landed awkwardly after jumping for a header in the first half of the 2-0 defeat away to Chelsea and had to receive treatment on his knee.
And Arsene Wenger revealed after the game that the Frenchman is almost certain to miss the midweek game.
“It is a little knee injury, ” said Wenger. “I don’t know how long it will be - certainly not for midweek. I don’t know yet how long.”
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The news comes as a blow to Arsenal, who will already be without Santi Cazorla and Gabriel after both players were sent off at Stamford Bridge on Saturday.
Per Mertesacker, who hasn't played since the win away to Crystal Palace last month, is likely to come back into the team while Alex Oxlade-Chamberlain could replace Cazorla.
Jack Wilshere, Danny Welbeck and Tomas Rosicky all remain out with long-term injuries.