The forward, who plays for Fulham, is expected to miss an extended period of time.
"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."The U.S.' World Cup opener is June 12 against England. They are already expected to be without Charlie Davies and Oguchi Onyewu won't return until the spring.
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.