Laurent Koscielny Subbed off with Back Injury vs. AC Milan

Arsenal defender Laurent Koscielny lasted just 11 minutes of Thursday's UEFA Europa League clash against AC Milan before he was substituted with a back injury, according to James Olley of the London Evening Standard. 

Per Olley, the centre-back appeared to suffer the injury after he tried to head the ball:

Calum Chambers replaced the France international at the Emirates Stadium.

Koscielny has been a regular in the Gunners defence for years and has made 24 starts in the Premier League this season. The 32-year-old also played the full 90 minutes in the first leg against Milan, his first minutes in Europe this campaign, per WhoScored.com.

He's one of Arsenal's most experienced defenders, and any long-term injury would be a huge loss for the team—but the problem is not believed to be serious.

Speaking after the game, via the club's official website, manager Arsene Wenger said: "He got a kick on his back. We don't think it's bad but he cramped up and he couldn't go on."

The Gunners are well behind their rivals in the race for a top-four finish in the Premier League, with the Europa League now the most likely avenue to a ticket for next year's UEFA Champions League.

Chambers is a solid replacement, while Rob Holding can also play in a central role if needed.