The defender went down in a heap unchallenged, and the fear is that he may have sustained serious damage to his Achilles tendon
Laurent Koscielny was forced off for Arsenal less than 10 minutes into the club's Europa League semi-final after suffering a bizarre apparent ankle injury.
The France defender went down unchallenged after appearing to lose his footing on the Wanda Metropolitana turf against Atletico Madrid.
His discomfort, however, was plain to see, as he grimaced on the floor while appearing to clutch his Achilles tendon.
Arsenal officials instantly recognised that Koscielny would play no further part in the game, and prepared Calum Chambers for an early first substitution.