England's Vunipola set to miss Six Nations
The English Premiership club confirmed the injury in a short statement on their website, leaving Jones to plan for a likely campaign without the experienced No.8.
"Saracens can confirm that as a result of an injury sustained in Saturday's European Rugby Champions Cup match against Ospreys, Billy Vunipola has fractured his forearm. Vunipola had an X-ray today confirming the injury," the club said.
"Billy is due to see a specialist later this week, when a timeframe for recovery may be defined. But until that time Saracens will make no further comment. Saracens will issue a further statement once Billy has seen a specialist."
Vunipola's lay-off comes just after he had made his comeback from a three-month absence with knee trouble, with the 15-15 draw against Ospreys being only his second game since returning.
The injury is a major blow for Jones, who will name his squad on Thursday for the Six Nations opener against Italy in Rome on February 4.
He is already braced for the unavailability of forward James Haskell, who was shown a red card in Wasps' 28-33 Champions Cup defeat to Harlequins on Saturday.
Tournament organisers European Professional Club Rugby will convene a disciplinary hearing for 32-year-old Haskell, which is likely to take place in the next few days. He could face a ban of up to six weeks.
Agence France-Presse
