Both players missed the 2-0 derby defeat at West Brom at the weekend with Scottish forward Fletcher suffering a torn calf in training ahead of the game.
McCarthy said Fletcher would miss a further two weeks with the problem while Foley underwent surgery on an ankle injury which will keep him out for at least a month.
Foley has only started one game this season - the Carling Cup victory at Northampton - and McCarthy said: "It depends on what type of operation he has as to how long he's out for.
"We always try conservative treatment but they're just going to open him up to see. I think it's only going to be a little clear-up operation so hopefully it will only be four weeks."
Elsewhere, defender Ronald Zubar is still nursing a groin injury having played for the reserves but Dave Edwards and George Elokobi are poised to return to training.
Source: Team Talk
Source: Team Talk