Arsenal boss Arsene Wenger fears Per Mertesacker suffered a serious ankle injury in his side's win at Sunderland.
Mertesacker was stretchered off after he appeared to catch his studs in the turf and collapsed to the ground in the build-up to Sunderland's goal from James McClean.
The German looked in considerable pain and Mertesacker will now have scans to determine the full extent of the injury.
Asked about Mertesacker's well-being, Wenger said "Not well, we fear ankle ligament damage.
"He will have a scan tomorrow to see how bad the damage is."
It seems highly unlikely Mertesacker will recover in time to be available for Arsenal's Champions League trip to AC Milan on Wednesday.