Billy Vunipola looks set to miss the entire Six Nations after Saracens confirmed the England number eight suffered a broken arm in their European Champions Cup victory over Racing 92.
Vunipola was hurt in the early stages of Sunday's contest at Allianz Park, with fears he had suffered a fracture that would lead to a lengthy lay-off.
In a statement on Monday, Saracens confirmed the 27-year-old had broken his arm for the fourth time in the last two years.
Although the European champions did not put a timescale on his recovery, it appears certain Vunipola will miss the Six Nations, which runs from February 1 to March 14.
Saracens' statement read: "Billy Vunipola has suffered a broken arm.
"He will see a consultant over the coming days to determine the full extent of the injury before undergoing rehabilitation."
The news also represents a significant blow to Sarries, who have endured a nightmare week with confirmation of their impending relegation from the Premiership due to salary cap breaches.
With the club set to play in the second tier of English rugby in 2020-21, the futures of their England internationals, including Vunipola, have come under scrutiny.