The Guardian is reporting the injuries to Alexis Sánchez and Santi Cazorla will keep both out of action for an extended spell for Arsenal this month.
For Sánchez, "intiial indications" have him out for three to four weeks with his hamstring injury as just 20 days separate today and Arsenal's headlining clash against Manchester City at the Emirates, Dec. 21. Meanwhile, the scans are back for Cazorla and indicate there is indeed damage to his knee ligaments. No indication of how severe that damage is, though, so we're still without a timetable for Santi's return.
The report also notes that Laurent Koscielny's hip wasn't as badly injured as feared, and he might even be available to feature this weekend at the Emirates against Sunderland.
Arsenal were already preparing to face off against Olympiakos in eight days without either Sanchez or Cazorla, both of whom add to an already length injury list for Arsenal including Francis Coquelin who won't be available for at least a few months.