Arsenal manager Arsene Wenger has confirmed that Laurent Koscielny will be unavailable for three weeks after picking up a hamstring injury in his side's 3-2 Champions League defeat at the hands of Olympiacos on Tuesday.

The France centre-back sustained the problem around the hour mark at the Emirates Stadium and was forced to leave the pitch as a result.

Koscielny has since undergone a series of tests to determine the extent of the issue and Wenger has now revealed he faces a spell on the sidelines.

"Laurent Koscielny is out for what looks like three weeks," the Arsenal boss said at a press conference ahead of Sunday's match versus Manchester United.

"Mathieu Flamini and Mikel Arteta will also miss out, but Francis Coquelin should be fit."

Koscielny's injury means Per Mertesacker is likely to return to Arsenal's starting XI for Sunday's encounter alongside Gabriel at the heart of the defence.

Koscielny is also set to miss next week's trip to Watford, and is doubtful for the crucial Champions League clash with Bayern Munich on October 20.