Clint Dempsey faces a long layoff after injuring his knee during Fulham's 2-0 loss at Blackburn on Sunday, the latest blow to American preparations for the World Cup.
The U.S. forward limped off after falling in the 62nd minute of the Premier League game. It wasn't immediately clear which knee Dempsey injured.
"Clint will be scanned tomorrow, 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," Fulham manager Roy Hodgson said.
"It's a concern up front because Clint Dempsey is very useful for us."
It's also a concern for the U.S., which begins its World Cup campaign in South Africa against England on June 12, then meets Slovenia six days later and Algeria on June 23.
U.S. team spokesman Michael Kammarman said the U.S. Soccer Federation training staff had been in contact with Dempsey, and the severity of the injury will not been known until after an MRI exam determines the exact ligament hurt and whether there is other damage.
The U.S. team is already likely to be without forward Charlie Davies in South Africa because of injuries sustained in a car crash last October, and defender Oguchi Onyewu is sidelined until spring following knee surgery.
The U.S. opens its 2010 schedule on Saturday against fellow World Cup qualifier Honduras in Carson, Calif., then meets the Netherlands in Amsterdam on March 3.
Dempsey's injury is also a major blow for Fulham, which is missing forward Bobby Zamora until March while he recovers from a shoulder injury. Dempsey had produced Fulham's best scoring chance Sunday in the first half at Blackburn when his overhead kick hit the crossbar.
"The doctors are pretty good these days — I would like to see them to be wrong more often," Hodgson said, trying to make light of the situation. "Often their initial diagnosis appears to be the correct one when the scan is made."