Arsenal boss Unai Emery fears Rob Holding faces a lengthy spell on the sidelines with a knee injury.
The Gunners defender, 23, was stretchered off in the 36th minute of their 2-2 draw with Manchester United on Wednesday evening.
Holding, who has started Arsenal’s last nine league games, is being assessed by club medics.
On Thursday, Emery said: “The first news is not good.”
Laurent Koscielny could fill the void left by Holding.
The former France defender, 33, has made two appearances for Arsenal’s Under-23s after recovering from a serious achilles injury.
Club captain Koscielny offers “experience, quality and personality” for the Gunners, according to Emery.
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He said: “The 18 can be for Saturday [against Huddersfield] and also we are doing one plan with him, maybe he can play against Qarabag [in the Europa League].
“He’s working with us every day in training and playing in the Under-23s with a good feeling. The next step, I think, is with us.”
Koscielny’s last competitive game for Arsenal came against Atletico Madrid in the Europa League semi-finals in May.