Maro Itoje could miss the start of England's Six Nations campaign after suffering a broken jaw during Saracens' 20-19 defeat to Harlequins on Sunday.
The British and Irish Lions lock was taken to hospital following a collision with Quins full-back and fellow England international Mike Brown where scans confirmed a fracture on his jaw.
Itoje will see a specialist later this week before his recovery timeframe becomes clear, but broken jaws can take up to 10 weeks to heal.
England's defence of their Six Nations title begins in less than nine weeks when they travel to Italy on Sunday 4 February.
The 23-year-old was rested for much of England's autumn schedule last month, starting only in their victory over Samoa, after his 2016-17 season was extended by Lions duty in New Zealand where he impressed in a drawn series.
Saracens will have to make do without Itoje as they attempt to recover from five losses on the spin, with consecutive home and away fixtures against last year's European Champions Cup finalists Clermont next on the schedule.