England and Saracens number eight Billy Vunipola will be out for 12 weeks with a fractured forearm, the Premiership club have confirmed.
Vunipola sustained the injury in Saracens' European Champions Cup draw with Ospreys last Saturday.
That all but ended his hopes of playing for England in the Six Nations and, after Vunipola saw a specialist, it has been revealed that his recovery will keep him out until April.
Champions Cup holders Saracens' hopes of progressing to the quarter-finals hang in the balance but, should they reach the last four, then Vunipola could return in time for the semi-finals.
"We will continue to monitor Billy's injury and of course provide the necessary support framework to get him back and fully fit," said a Saracens spokesman.
He will have to play the role of spectator during England's bid for a third successive Six Nations title, with the tournament starting on February 3 and coming to a conclusion on March 17.
Saracens are second in the Premiership, five points behind leaders Exeter Chiefs with 13 games played.
However, their form in Europe has been less consistent. Saracens are third in Pool 2, five points behind leaders Clermont Auvergne and two adrift of Ospreys, with just a home match against Northampton Saints left to play.