England forward Courtney Lawes is a doubt for the autumn international with South Africa on 3 November because of a recurring back problem.
The 29-year-old Northampton Saints player suffered a back spasm on 6 October after sleeping in a hotel bed that was ill-suited to his 6'7" frame.
He has left the training camp in the Algarve for specialist UK treatment.
England are already without the injured Chris Robshaw, Joe Launchbury and Billy and Mako Vunipola.
Lawes first experienced back problems on the day of Northampton's defeat by Leicester, and was forced to withdraw during the warm-up to the game.
England forwards coach Steve Borthwick said: "He is expected to return to Portugal early next week. He has taken part in modified training this week."
The South Africa match begins a month of Saturday internationals at Twickenham for England, with other matches against New Zealand, Japan and Australia.