Arsenal's Koscielny leaves match vs. Olympiacos with apparent hamstring injury

Arsenal will need to mount a comeback against Olympiacos without its best defender, after Laurent Koscielny hobbled out of Tuesday's contest in the 57th minute clutching his right hamstring.

The Frenchman seemed to sustain the injury while making a slide tackle early in the second half, and after playing on for a few minutes, gestured to the sideline that he needed to be replaced.

Veteran centre-back Per Mertesacker came into the match in his stead.

With matches against Manchester United and Bayern Munich on the horizon, Arsene Wenger can ill afford to be without Koscielny for an extended period of time.