Billy Vunipola suffered a fresh injury setback as he failed to emerge for the second half of Saracens' 15-15 Champions Cup draw with the Ospreys.
England number eight Vunipola only returned from three months out with a knee issue last weekend, but a wrist injury curtailed his involvement in Swansea.
Defence coach Alex Sanderson revealed during an interview with BT Sport that Vunipola - who also missed the British and Irish Lions tour of New Zealand with a shoulder injury last year - "can't feel his wrist", raising further questions over whether he will play a part in England's Six Nations campaign.
Speaking after playing 70 minutes against Wasps last week, Vunipola said: "It means a great deal [to come back] and I feel I'll be better for it next week and hopefully, if I can keep going like this, I'll be back to where I was in no time.
"It can be frustrating being injured but there can be higher powers at work. It taught me a huge lesson about looking after myself better in terms of recovery and sleep. I thought I did that better this time. Whatever runs for me, I'll just try to smash it.
"If I can convince [England coach] Eddie [Jones] I am in the right shape and playing well enough to be picked in his squad, then great.
"I want to play for England as much as I can and I have missed it but right now my biggest focus is playing well for Saracens and hopefully transferring that on to the pitch at Twickenham."
Meanwhile, England flanker James Haskell was sent off for a dangerous tackle on Jamie Roberts in Wasps' defeat to Harlequins and a ban could render him unavailable for selection by Jones.