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.
And next up for Arsenal is a home clash with title rivals Chelsea next Monday.
