Back row forwards James Haskell and Jack Clifford did not fly to Vilamoura, Portugal on Monday to join-up with England's training squad.
Both players sustained injuries in their club games on Sunday and were due to arrive in Portugal today (Monday 23 January) 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 was in action in Wasps' Champions Cup victory over Zebre in Parma and Clifford played for Harlequins in their Challenge Cup loss against Stade Français in Paris.
The pair missed England's end-of-year series through injury and were both named in Eddie Jones' 34-man Six Nations squad on Friday.
England open their campaign against France on 4 February at Twickenham Stadium.
Head coach Jones picked three uncapped players in his squad.