Billy Vunipola is a doubt for England’s clash with Fiji on Saturday, while Dave Attwood has definitely been ruled out.
World player of the year nominee Vunipola, who forced Springbok lock Eben Etzebeth to withdraw from England's 37-21 victory over South Africa at the weekend following a clash of heads, will be unable to train on Monday or Tuesday.
A Rugby Football Union spokesman was unable to clarify the nature of the injury except to describe it as a "knock" - but it places a question mark over his availability for the second autumn international.
If Vunipola fails to prove his fitness in time for Saturday, then Nathan Hughes will start at number eight in what would be his full debut after he won his first cap off the bench against South Africa.
Bath second row Attwood is definitely out of the match at Twickenham on Saturday, live on Sky Sports 2, after injuring a knee in the win over South Africa and will return to his club having received treatment at the squad's Surrey training base.
England have opted against drafting Josh Beaumont into the squad as cover for Attwood and will instead rely on Charlie Ewels, who is already with the team.
England forwards coach, Steve Borthwick, said: "We've got a few bumps and bruises from Saturday.
"Dave Attwood is having his knee treated. He's having an injection on his knee and is then returning to Bath, so is unavailable."
Eddie Jones has led England to 10 wins from 10 since he was appointed head coach but Borthwick has warned it will not be easy to make it 11 from 11 against Fiji this weekend.
"Whatever anyone else says doesn't really matter. We are concentrating on ensuring our performance improves. That is the quality of our preparation; our tactical preparation, our physical preparation, our mental preparation, and helping our players improve," he told Sky Sports News HQ.
"This is my role as a coach, helping our players improve so they are better players this week than they were last week. That's the aim so, whatever anyone else says, we are concentrating on improving and making sure we produce the best 80 minutes we possibly can."