Mako Vunipola: Saracens and England prop could miss Six Nations
England prop Mako Vunipola's Six Nations involvement is in doubt after he injured a knee during Saracens' win at Sale last Sunday.
The 25-year-old could be out for four to 12 weeks depending on the severity of ligament damage in the joint.
A month's absence would put him on track to play in England's match against France on 4 February.
But a longer-term lay-off would rule him out beyond their final match, against Ireland on 18 March.
His brother Billy, a team-mate at club and international level, is likely to miss the Six Nations after having surgery on a knee injury in November.
Following Saturday's 21-6 win over Newcastle, Saracens director of rugby Mark McCall said: "It's not looking good [for Mako] to be honest.
"He's got to go and see his consultant in four weeks' time, and then we will determine how long he will actually be out.
"We have to let the injury settle down as much as possible to have any idea because otherwise you are just guessing.
"It's a big blow to lose Billy and then Mako. Both are such important players or us. We were going to lose them for the Six Nations but to lose them in the run-up to that is so disappointing."
Mako Vunipola played in every game of England's 2016 Grand Slam, as well as starting each of the summer Tests - a 3-0 whitewash of Australia - and the autumn internationals against South Africa, Fiji, Argentina and Australia.
If he does miss out, Harlequins' Joe Marler is likely to come into the England front row, though Leicester's Ellis Genge is also in contention.
Vunipola will definitely miss Premiership leaders Saracens' festive programme, which includes matches against Newcastle on Saturday and Leicester on 1 January.
For the latest rugby union news follow @bbcrugbyunion on Twitter.