Billy Vunipola could miss England's Six Nations campaign after suffering a suspected broken arm during Saracens' Champions Cup match against Racing 92 on Sunday.
N0.8 Vunipola, who featured in every game in England's run to World Cup final last year, was injured during the early stages of Saracens' 27-24 victory, and was withdrawn in the sixth minute at Allianz Park.
"They suspect a broken arm," Saracens director of rugby Mark McCall said. "I hope it's not. I don't think it's been confirmed yet."
England head coach Eddie Jones will name his Six Nations squad on Monday and will be monitoring Vunipola, who could be out with a serious arm injury for the fourth time in two years.
Vunipola's club future is uncertain after it was confirmed Saracens will be relegated from the Premiership at the end of the season for persistent salary cap breaches.