Less than 10 minutes into his club's Europa League semi-final second leg at Atletico Madrid, the centre-back went down unchallenged after appearing to lose his footing on the Wanda Metropolitano turf.
His discomfort was plain to see, as he grimaced on the floor while clutching 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.
Koscielny received limited treatment on the pitch and was then stretchered off, with the game still poised at 1-1 on aggregate after Arsenal and Atletico played out a score draw in the first leg.
Goal had it confirmed that the club believes Koscielny has torn his Achilles tendon and faces a prolonged spell on the sidelines.
The defender's season has thus come to an end, and he will miss the rest of the Gunners' games.
It could even be his last appearance at the club, with Arsenal expected to look for defensive reinforcements this summer in an effort to improve their struggles in 2017-18.
France too will miss their experienced centre-back, with the World Cup just a month away.
Torn Achilles for Koscielny. — Chris Wheatley (@ChrisWheatley_) 3 de mayo de 2018
Arsenal boss Arsene Wenger confirmed after the match that the club fear the worst, though says he is hoping for "a miracle" for Koscielny when the defender has the injury scanned.
"It doesn't look very good," said the Frenchman. "His Achilles, you can always hope for a miracle on the scan, but the first signs are not good. It looks like it could [be ruptured]."
Koscielny's team-mate Pierre Emerick-Aubameyang instantly took to Twitter to send a message of support, saying: "Damn it, courage bro".
The 32-year-old's absence is a big setback for Les Bleus, who have been touted as one of the favourites for the tournament in Russia.