Billy Vunipola will miss the next four months of Saracens' Premiership season and England's November internationals with a knee injury.
Vunipola suffered the injury during Saracens' 41-13 demolition of Sale Sharks on Saturday, the back-row limping out of the one-sided win in the first half at Allianz Park.
Scans revealed damage to the meniscus in his right knee that required immediate surgery, Vunipola going under the knife on Sunday to correct the issue.
Saracens revealed the 24-year-old's period of rehabilitation will be 16 weeks, meaning he will miss a large chunk of the club's domestic and European fixtures.
He will also not feature for England later this year against Australia, Argentina and Samoa, and will have not played much rugby heading into the Six Nations.
"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," said Sarries director of rugby Mark McCall.
"It's desperately disappointing for him, considering how hard he has worked during the offseason to get back to full fitness, but Billy is a resilient character and he'll have a strong support network around him to get him back on the field in the best possible shape.
"What's now important for Billy is his recovery and we look forward to his return later this season."
It is the latest injury in a disrupted year for Vunipola after he missed four months at the start of 2017 and has only just returned from the shoulder surgery that saw him miss the British and Irish Lions tour to New Zealand.