The 2007/08 Guinness Premiership Player of the Year will have surgery on his ankle after sustaining ligament damage during Gloucester's Heineken Cup defeat against Cardiff last Sunday.
"On behalf of the squad my thoughts are with James," said England team manager and former Lions skipper Martin Johnson.
"The coaching and medical teams from Gloucester and England will be in regular contact with him during his recovery and rehabilitation."
Gloucester head coach Dean Ryan reiterated those sentiments: "To lose a player of James's quality is obviously a massive blow to us. He is an outstanding player and central to our attacking threat.
"Considering the other injuries we have across our backline this could not have come at a worse time for us. Naturally, he will get all the support he needs in his recovery process."