Brian McBride is expected to be sidelined until Christmas after undergoing knee surgery in America.
The Fulham captain suffered a dislocated patella whilst scoring Fulham's opening goal in last Saturday's 2-1 defeat to Middlesbrough.
He has flown back to his homeland, and is expected to face a four-month lay-off during which he will be put through intense rehabilitation.
Manager Lawrie Sanchez said: 'He is having an operation and we expect him to be out until Christmas.
'He is captain, he is a centre forward who wins a lot in the air and is a big presence in the dressing room.
'So he is quite a big loss for us.'
Sanchez is particularly disappointed to lose his captain after he had begun to work well with new strike partner David Healy.
Sanchez added: 'They had an understanding going. They work for each other and David can read Brian's flick-ons.
'It is disappointing, particularly in the action of scoring his first goal of the season when it was all flowing.'
Fulham will now attempt to sign a new forward before the transfer window closes on August 31.
They have had a £1.5million bid for Watford's Darius Henderson turned down, and Sanchez said: 'We are looking for new players, I still think the squad is thin despite what has been said about how much has been spent.
'We lost seven first teamers from last year and another couple of signings would certainly help the equation. We are looking for a big centre forward.'
Meanwhile, Papa Bouba Diop and Liam Rosenior are still waiting to find out where they will spend the next part of the season.
Both players are available for transfer, but could be forced to stay at Craven Cottage if no club makes a firm offer by the time the transfer window shuts next week.
Sanchez explained: 'Our players do have a value and nobody has come near to that in any shape or form.
'We haven't had offers (for Diop) - we have had enquiries. We have had absolutely nothing on Liam at this moment in time.'