With Arsenal leading Nottingham Forest 2-0 after 49 minutes, Timber pulled up sharply off the ball.
It was only his second appearance for the Gunners, having playing in the Community Shield win over Manchester City ahead of the start of the season.
But concerningly he was unable to finish the match, pulling up and then being forced to leave the field with the injury.
It had looked like Timber sustained an issue in the first half following a coming-together with Brennan Johnson, but he was able to re-emerge after the break before having to come off.
“The doctors looked at him and they were happy for him to continue, he was as well,” Arteta said when asked if he could have taken Timber off sooner. “Straight away in the first action of the second half he made a movement and it was a bit funny. We took him out straight after and now we have to assess him and see what he has.”
The former Ajax defender, who joined the Gunners this summer for £34 million to further bolster their defence, was replaced by Takehiro Tomiyasu, suggesting that Gabriel Magalhaes may also be suffering from an undisclosed injury.
Timber was one of a number of summer signings made by manager Mikel Arteta, adding to Kai Havertz from Chelsea, and club record signing Declan Rice from West Ham for £105 million.
Arsenal’s start to the campaign was already impacted before the start of play, when kick-off was delayed for half an hour due to a turnstile issue.
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