Saracens and England scrum-half Richard Wigglesworth will miss the remainder of the season after undergoing knee surgery.
The 28-year-old sustained the anterior cruciate ligament injury during Saracens' LV=Cup win over Exeter Chiefs last Sunday.
"It's a big blow for Wiggy, who has been playing so well for Saracens and has worked so hard in the past 18 months to get himself back in the England side," director of rugby Mark McCall said.
"In my opinion he was one of England's stand-out players at the World Cup and it's a shame he will miss the Six Nations.
"It's also a blow for us but we are lucky to have two very good players in Neil de Kock and Ben Spencer who have both performed so well this season and because of that, we are not looking to recruit."
The 28-year-old picked up the injury in his first match after England's World Cup campaign.
