Fulham's American winger Clint Dempsey has suffered suspected cruciate knee ligament damage, which could jeopardise his place at the 2010 World Cup.
The 26-year-old United States international was injured in the second half of Fulham's 2-0 loss to Blackburn and went to hospital for a scan.
"The initial prognosis for Clint is not good," said Fulham manager Roy Hodgson.
"It looks like he has suffered cruciate knee ligament damage, which if he has, keeps him out of action for a while."
Dempsey has been a key player for Fulham this season, making 29 appearances and scoring seven goals, and before he was injured he hit the crossbar with a spectacular overhead kick.
"We won't know for certain until the scan is completed on Monday but the doctors are pretty good these days and often their initial prognosis proves to be the correct one," added Hodgson.
Dempsey has played 44 times for the US, who play England in their opening Group C game in the World Cup in South Africa this summer.