The 28-year-old midfielder suffered a calf injury in last weekend's 2-0 defeat at Manchester City and will not be fit in time for the game at Old Trafford, although he is likely to return to training next week.
Mahamadou Diarra is not likely to play again for a month because of a knee injury whilst Kerim Frei is also absent because of a pelvis problem.
Cottagers boss Martin Jol insists he won't make wholesale changes - despite the Premier League being the club's priority - given that his squad is not big enough.
And, while he accepts his side will be the underdogs going into the game against the Red Devils, he is hoping for a bit of good fortune which could see them pull off a shock.
"You need to have that confidence that you can get a result or you may as well stay at home," he said in his pre-match press conference.
"Hopefully we can take them back to Craven Cottage but, if it is a one-off, with a little bit of luck we could go to the next round."