Arsenal defender Ainsley Maitland-Niles has suffered a "small fracture" to his left leg and will be out for six to eight weeks.
Maitland-Niles, 20, had been expected to a regular in the first-team squad this season following a promising previous campaign.
However, after suffering the injury during the first half of the opening Premier League match against Manchester City, Maitland-Niles could now be set for up to two months of rehabilitation.
Arsenal lost 2-0 against champions City in new head coach Unai Emery's first league game in charge.
The Gunners head to Chelsea on Saturday, and provided an injury update on Thursday morning, ahead of Emery's scheduled press conference.
Arsenal reported left-back Nacho Monreal and Danny Welbeck were both "in full training", so should come into contention for the trip to Stamford Bridge.
Defender Carl Jenkinson is, however, also expected to be out for six to eight weeks after he "sustained a significant sprain to his right ankle", the club confirmed, which is likely to mean the player will not be leaving on loan to a Sky Bet Championship club.
Sead Kolasinac should return to training from his knee problem in October, while club captain Laurent Koscielny continues his recovery from Achilles surgery.
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