Koscielny was taken off on a stretcher 42 minutes into Arsenal's 6-3 loss at Manchester City on Saturday after suffering a nasty gash to his knee in his unsuccessful attempts to prevent Alvaro Negredo scoring the hosts' second.
Manager Arsene Wenger fears the 28-year-old could be sidelined for some time, but stopped short of putting an approximate timeframe on the injury rehabilitation until Koscielny is properly assessed.
"He has a cut and he will be out for a while, but for how long I don't know," said Wenger.
Losing the services of a player as influential as Koscielny – who has started 15 of Arsenal's 16 Premier League games this season – compounded what was a miserable afternoon for Wenger's men.
Defeat at the Etihad Stadium coupled with Chelsea's win over Crystal Palace reduced the club's lead at the top of the table to two points.
