Maro Itoje is a doubt for the start of England's Six Nations campaign after Saracens confirmed the Lions lock sustained a fractured jaw in his side's Premiership defeat by Harlequins on Sunday.
Itoje is understood to have undergone an operation on the jaw after scans on Monday revealed he has suffered a fracture after a collision with Harlequins full-back Mike Brown, his England colleague in the 48th minute.
Itoje must see a second specialist on Thursday after the swelling had eased to determine the length of his rehabilitation. It is thought that the fracture may not be too serious but even if that is the case the 23 year-old could be left with a race against time to prove his fitness for England's opening game against Italy in Rome on Feb 4.
Anthony Watson, Itoje's England team-mate, was ruled out for two months and missed the entire autumn Test campaign in 2016 when the Bath wing sustained a similar injury during national training camp in Brighton.
England are due to travel to Portugal for a warm-weather training camp in mid-January and if Itoje is unable to take part by then it would leave him struggling to be considered for selection for the Italy game.
The injury to Itoje, who was a nominee for World Rugby player of the year in Monte Carlo last week, also comes as a massive blow to Saracens, who have dropped to third place in the Premiership table after three successive league defeats.
Mark McCall's side face back-to-back Champions Cup matches against Clermont Auvergne in the next two weekends that will have a major impact on their hopes' of qualifying from pool two to the knock-out stages.
England at least have plenty of cover in the second row, with Joe Launchbury and Courtney Lawes both excelling during the three autumn Test victories over Argentina, Australia and Samoa while Itoje was eased back into the side after undergoing a post-Lions conditioning programme and made his only start in the back row against Samoa.