England's injury problems have deepened before the Six Nations after Owen Farrell and Kyle Eastmond were hurt in Sunday's European Champions Cup games.
Saracens boss Mark McCall said it was too early to tell if Farrell is out of the Six Nations opener against Wales.
"It's a new injury and we don't know the extent of it just yet," he said.
Speaking to the Daily Telegraph, he added: "It's complicated with knee injuries. Sometimes you can do that [walk off the pitch] and it's more serious, but we'll get him looked after and we'll know in the next few days."
Bath inside centre Eastmond is to have a scan to find out the extent of his injury.
A 34-man England training squad will meet on Monday, with Stuart Lancaster's men set to open their Six Nations campaign away to Wales on 6 February.
England forwards Geoff Parling and Tom Wood were injured during Saturday's matches, joining a growing injury list.
|Ben Morgan (leg), Ben Foden (knee), Manu Tuilagi (groin), Courtney Lawes (ankle), Geoff Parling (knee), Tom Wood (ankle), Joe Launchbury (neck), Ed Slater (knee), Eastmond (shoulder), Farrell (knee)|
The second row is likely to be an area of particular concern for Lancaster, who is already without locks Joe Launchbury (neck injury) and Ed Slater (knee) for the entire tournament, while Courtney Lawes has had minor ankle surgery which has ruled him out until the middle of the tournament at the earliest.
Leicester director of rugby Richard Cockerill anticipates Parling could be sidelined for "three to four weeks" with damage to his medial cruciate ligament, which if confirmed would likely see Parling miss at least the opening Six Nations game against Wales. "It is not too bad, it is nothing too drastic," Cockerill added.
Parling's uncapped Tigers team-mate Graham Kitchener and Saracens' George Kruis - who won his first four caps off the bench in the autumn - will compete to start in Cardiff alongside Bath's Dave Attwood.
Gloucester number eight Ben Morgan will miss the entire Six Nations with a leg injury, as will Northampton full-back Ben Foden (knee).
Leicester centre Manu Tuilagi (groin) will miss the Wales game, but it is hoped he may recover in time to play some part in the tournament.
There was further bad news for Lancaster with England hooker Dylan Hartley receiving a yellow card in the Northampton defeat in the wake of a series of issues with his on-field discipline.
"I knew the spotlight would be on me," said the hooker, who is understood to be seeing a sports psychologist.
"A player runs over me, I put my arms up and tackle him, and the next thing I know I am watching the game."