England and Saracens man Billy Vunipola faces race to be fit for 6 Nations after being ruled out for four months

England number eight Billy Vunipola faces a race against time to be fit for the start of the RBS 6 Nations after undergoing knee surgery.

Vunipola has been ruled out for 16 weeks with the cartilage damage sustained in Saracens' 41-13 Aviva Premiership victory over Sale at Allianz Park on Saturday and will miss the autumn series against Argentina, Australia and Samoa.

"For any team to lose a player of the ability of Billy is a blow and he will be missed through the autumn for both Saracens and England," Saracens director of rugby Mark McCall said.