Rob Holding was picked to play at Manchester City on Saturday - but didn't start the game.
The centre back felt a tightness in his hamstring during the warm-up and his place was taken by David Luiz.
After the game, Mikel Arteta gave an update.
"We don’t know [how bad it is]," he said.
"But it was bad enough to make the decision to get Rob out of the game and to get David in.
"David was really focused, he came on and I think he had a really good game."
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