The powerful back rower has 34 England caps, but last appeared for the global team in March after a four-month lay off with a knee injury.
England's players are training together in Oxford, northwest of London, for three days as they continue their preparations for November Test matches against Argentina, Australia and Samoa.
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.
"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".
He withdrew from the Lions tour to New Zealand in May with a shoulder injury and subsequently missed the start of Saracens season.