Saracens director of rugby Mark McCall revealed that England No 8 Billy Vunipola is having a scan on his knee after leaving the field injured during the 41-13 win over Sale.
Vunipola received lengthy treatment just before half-time after being hurt as Saracens defended resolutely under a Sale barrage before getting to his feet and hobbling around the perimeter of the ground, aided by two medics.
Harlequins No 8 Jack Clifford has been added to England's training squad, which convenes in Oxford on Sunday, as a replacement for Vunipola.
McCall said: "It didn't look great and I hope it's not as bad as it looked but he's gone for a scan.
"It's the same knee he injured for England against Argentina last November and it's cruel luck as we obviously feel for him terribly."
Vunipola's injury is the latest in a host of injuries for Saracens' back row players.
"Michael Rhodes has just had surgery on his shoulder and will be out for 12 weeks. Schalk Burger will be out for four weeks with another shoulder injury," McCall said.
"We have some good academy players, who can fill in but it is likely that we will have to bring someone in."
Despite Vunipola's absence, Saracens controlled the game by scoring five tries. Ben Spencer scored two of them, Liam Williams, Nathan Earle and Vincent Koch the others, with Owen Farrell converting all five and adding two penalties to leave McCall satisfied.
"It was job done as we had a great start to the match with a brilliant first 20 minutes," he said.
"We were extremely physical in defence but a couple of things frustrated us and we were disappointed to have conceded a number of line-breaks and two tries against us."