The Gunners star suffered the injury in the midweek Champions League play-off win over Fenerbahce.
“His hamstring is much more damaged than we thought at the start,” Arsenal boss Arsene Wenger said.
“It is very disappointing and we have paid a very heavy price for that win. We have to be patient now.”
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Since Guus Hiddink arrived at the helm, Chelsea have improved considerably after a tricky start to the campaign.
Newcastle United are staring down the barrel of relegation to the Championship for the second time in the past eight years.