Arsenal midfielder Ainsley Maitland-Niles will be out for six to eight weeks after suffering a leg fracture in the loss to Manchester City.
Right-back Carl Jenkinson will also be out for up to two months with an ankle sprain, seemingly ruling out the possibility of him being sold before the European transfer window closes.
Maitland-Niles filled in at left-back in the 2-0 loss to City but limped off in the first half, and Arsenal said Thursday he has "a small fracture to his left fibula."
However, Nacho Monreal has been in full training this week and could be ready to return against Chelsea on Saturday, with fellow left-back Sead Kolasinac not expected back in training until October because of a left knee injury.
Arsenal also said captain Laurent Koscielny is now expected back in training in November, with earlier estimates having aimed for a December return. Koscielny tore his Achilles tendon toward the end of last season.
The rarely-used Jenkinson had been free to leave the club this transfer window but his latest injury means he is expected to remain at Arsenal until at least January.
Meanwhile, Arsenal have sent academy defender Krystian Bielik to League One side Charlton on loan for the season to gain more first-team experience.
It's the third loan-spell for the 20-year-old Bielik, whose time at Walsall last season was ruined by a long-term injury.
The Poland youth international joined the Gunners in 2015 as a promising holding midfielder but has largely been used as a centre-back at Arsenal, where he made two League Cup appearances under Arsene Wenger.
He spent the 2016-17 season on loan at Championship side Birmingham, playing in 10 league games.
"We would like to wish Krystian well for his loan at Charlton Athletic this season," Arsenal said in a statement.