The centre-defender twisted his right ankle and is likely to miss Monday's friendly against Nagoya Grampus Eight
The injury didn't look so serious but Arsenal's assistant manager, Steve Bould had to continue in his position for the training session to continue. Koscielny adds to Arsenal's injury list as Thomas Vermaelen is out for three months with a fracture in his back.
Elsewhere, it had been reported that Nacho Monreal has also sustained a back injury and would be out till October.
Szczesny also sat out of training due to a back strain and might also miss tomorrow's game.
Giroud didn't begin training with the rest due to a knock but later joined in the session.