London - England No 8 Billy Vunipola faces "approximately 12 weeks" on the sidelines after breaking his arm, according to his club Saracens.
Powerhouse back-row forward Vunipola fractured his forearm playing for title-holders Saracens in their 15-15 European Champions Cup draw with Welsh side Ospreys last weekend.
It was immediately feared that Vunipola would miss the whole of champions England's upcoming Six Nations campaign and the 25-year-old was duly left out of national coach Eddie Jones's squad for the tournament opener against Italy in Rome on February 4, with the Australian saying on Thursday: "I don't think we'll see Billy (in the Six Nations)."
Friday saw London club Saracens confirm the severity of the luckless Vunipola's latest injury following a consultation with a medical specialist.
"On Sunday (14th January), Saracens confirmed that Billy Vunipola had fractured his forearm in the European Rugby Champions Cup match against Ospreys," said a club spokesperson.
"As a result of this, and having now seen a specialist, we can now advise that Billy's recovery will take approximately 12 weeks.
"We will continue to monitor Billy's injury and of course provide the necessary support framework to get him back and fully fit."
Vunipola, capped 34 times by England, had only recently returned from three months out with knee trouble, having missed last year's British and Irish Lions drawn series in New Zealand because of a shoulder problem, when he suffered his latest setback.
Being ruled out for a further 12 weeks - his fourth major injury in the past 14 months - would see Vunipola, whose brother Mako is an England prop, miss the whole of this season's Six Nations given Jones's men finish the tournament against Ireland at Twickenham on March 17.
Vunipola's latest injury is not the only back-row problem confronting England coach Jones.
He will also be without Nathan Hughes (knee) for the early rounds, while James Haskell has been ruled out of both the Italy match and England's fixture against Wales at Twickenham the following week after being given a four-week ban for being sent off for a dangerous tackle on Wales centre Jamie Roberts during Wasps' defeat by English rivals Harlequins.
It all leaves the likes of Bath's Zach Mercer and Exeter's Sam Simmonds competing for a place in England's back row.