England lock Maro Itoje fractured his jaw playing for Saracens against Harlequins on Sunday, a scan has shown.
The 23-year-old, who collided with Mike Brown, will visit a specialist this week to determine when he can return.
England begin the defence of their Six Nations title in less than nine weeks, away to Italy on Sunday, 4 February.
Itoje, who has 14 caps, started every match as Eddie Jones’ side won the 2017 title, but only played in two autumn Tests as his playing time was managed.
After a heavy schedule that included starring for the Lions as they drew their series 1-1 away to New Zealand in the summer, the versatile forward was rested for England’s opening fixture against Argentina, came off the bench against Australia and started the victory over Samoa.
He was forced off in the 48th minute of Saracens’ 20-19 Premiership defeat by Quins at The Stoop.
Sarries play Clermont home and away in the Champions Cup over the next couple of weekends, before visiting Leicester Tigers in the Premiership on 24 December.