The 20-year-old Maitland-Niles has a fracture to his left fibula. Defender Carl Jenkinson has also suffered a sprain on his right ankle and will be out for upto two months.
"Sead has a left knee injury. We are aiming for him to return to training in October," Arsenal said on their website.
France international Laurent Koscielny, who missed his nation's World Cup-winning campaign with an Achilles tendon tear, is on the mend and is expected to return to training in November.
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Arsenal have been boosted with striker Danny Welbeck and defender Nacho Monreal participating in full training ahead of Saturday's league match against Chelsea.
(Reporting by Aditi Prakash in Bengaluru; editing by Sudipto Ganguly)
