Arsenal midfielder Aaron Ramsey is set to be out for up to six weeks with a hamstring injury, Goal understands.
The Gunners' midfield talisman was diagnosed on Monday with a grade two hamstring strain sustained in his side's 1-1 draw with Tottenham last Saturday.
The Welshman will be joined on the sidelines by Mikel Arteta, who will miss two to three weeks with a grade one calf strain.
The absence of Ramsey until mid-November and Arteta until after the October international break is a huge blow for Arsene Wenger ahead of fixtures this week against Galatasaray on Wednesday and Chelsea on Sunday.
Goal has learned that key defender Laurent Koscielny is also a doubt for the Champions League match against Galatasaray with a long-running Achilles tendon problem.
The centre-back faces a late fitness test on the injury, while Jack Wilshere will learn on Tuesday the extent of the ankle problem he sustained in the north London derby.
Initial tests have suggested Wilshere's injury is not too serious but Arsenal are likely to be cautious with the England man given his history of ankle trouble.
The lengthy absence of Ramsey is a particularly grievous blow to Wenger given how badly he was missed when sidelined with a thigh injury for most of the second half of last season.
The midfielder's injury is not the most severe type of hamstring strain but he has a partial tear of the muscle and will miss the whole of October.
The Arsenal manager will also be conducting further investigation into why his squad still seem so susceptible to injuries, with the muscular problems suffered by Ramsey and Arteta of particular concern after they were rested from the Capital One Cup defeat to Southampton.
"We have to really analyse what happened," Wenger said on Saturday. "There's no logic as they had quite a light week for them. I don’t understand."
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