London - Harlequins No 8 Jack Clifford was called into England's training squad on Sunday after Billy Vunipola injured his knee during Saracens' win over Sale on Saturday.
Vunipola had to be helped from the pitch after hurting his right knee in the first half of Saracens' 41-13 victory and is now awaiting the results of a scan.
England's players are training together in Oxford, northwest of London, for three days as they continue their preparations for November Test matches against Argentina, Australia and Samoa.
"This camp in Oxford is a really positive opportunity for the group to come back together and sharpen our focus and preparation ahead of our first match against Argentina," head coach Eddie Jones said in a press release.
Clifford has been capped 10 times by Jones, but appeared to have fallen out of favour with the Australian.
Vunipola, 24, was playing in only his second match after returning from a shoulder injury that kept him out of the British and Irish Lions' tour of New Zealand.
He injured the same knee against Argentina last November, which kept him on the sidelines for three months.
"Billy has had some cruel luck with injuries so we're all praying and hoping that this isn't too serious," Saracens director of rugby Mark McCall said.