Exeter Chiefs' Henry Slade has not travelled to Portugal for England's training camp ahead of the Six Nations.
Slade will remain with his club for further assessment on the shoulder injury he sustained during the 28-21 defeat to Glasgow Warriors on Saturday.
The 24-year-old playmaker could miss the start of England's defence of their Six Nations title on February 4.
Saracens' Billy Vunipola and Wasps' Nathan Hughes have both already been ruled out of the Six Nations with injuries, leaving head coach Eddie Jones short of ball-carrying back-rows.
Despite selection headaches, Jones has maintained England will have no excuses if they fail to win the competition.
England begin their Six Nations campaign against Italy on February 4 and finish against Ireland on March 17.