England and Saracens number eight Billy Vunipola has been injured and will be out for four months with a knee cartilage injury.
The 24‑year‑old underwent surgery on his right knee on Sunday and will now begin rehabilitation.
Vunipola will miss England’s autumn Tests against Argentina, Australia and Samoa so head coach Eddie Jones has added Harlequins' Jack Clifford, 24, to his training squad.
He withdrew from the Lions tour to New Zealand in May with a shoulder injury and subsequently missed the start of Saracens' season.
In addition to missing England's autumn campaign, Vunipola will also be absent for the start of Saracens' European Cup defence.
