James Haskell and Jack Clifford face medical assessments ahead of England's Six Nations opener with France next weekend, after suffering injuries on club duty.
Haskell, who only returned to action earlier this month following a lengthy lay-off brought about by major toe surgery, and fellow back-row forward Clifford were each named in a 34-man squad by Eddie Jones last week.
However, on Monday, it was confirmed the duo will remain in the UK rather than attending a training camp in Portugal.
A statement from England Rugby read: "Jack Clifford [Harlequins] and James Haskell [Wasps] will not fly to Portugal to join up with England's RBS 6 Nations training squad.
"Both players sustained injuries in their club games on Sunday and were due to arrive in Vilamoura today [Monday] but will instead stay in the UK to undergo further medical assessments, after which a decision will be made on when they join up with England."
Haskell, whose initial comeback for Wasps was cut short by a head injury, started his club's European Champions Cup tie against Zebre on Sunday.
After that match, the 31-year-old told BBC 5 Live's Rugby Union Weekly podcast: "I tried to do the basics well. I've obviously got a bit of discomfort in the foot, but you are always going to have that."
England head coach Jones has already seen his back-row options depleted by the loss of ex-skipper Chris Robshaw to injury.
Saracens star Maro Itoje, highly influential at lock in his international career to date, is believed to be in contention to start at number six against France.