Clint Dempsey's World Cup hopes could depend on the results of a Monday scan after the United States forward limped off in the second half of Fulham's 2-0 Premier League defeat at Blackburn.
Dempsey is a key man for the States, who begin their World Cup campaign in South Africa against England on June 12.
Fulham boss Roy Hodgson, who is also without striker Bobby Zamora, said: "Clint will be scanned on Monday but we feel it might be a posterior cruciate knee ligament which would be very bad for us and he would be out for a while.
"It's a concern up front because Clint Dempsey is very useful for us but now with both he and Bobby out it makes us a bit weaker in that area than we'd like to be.''
However Hodgson admitted his growing injury list - also including Paul Konchesky who was pulled off with an ankle problem at half-time - will not lead to a transfer window spree.
Hodgson added: "It's a very difficult market at the moment. There's not many players who are moving about and we will be very limited in what we can spend.
"I'm a little bit reluctant to go and spend money on a player who maybe in the long term might not make us better. We will get players back and maybe we just have to accept that.''