Wayne Rooney limped out of Manchester United's 2-1 Champions League defeat to Bayern Munich on Tuesday with an ankle injury.
The England striker put United ahead in the second minute with his 35th goal of the season, but Bayern went ahead in injury time before Rooney twisted his right ankle while competing for the ball with Mario Gomez.
Rooney immediately signaled to the United bench that he had to come off the field and hopped from the field, leaning on the shoulder of one of United's medical staff.His right boot was off.
"He got a kick in the ankle," United manager Alex Ferguson said.
"We'll have to wait and see tomorrow."
The injury comes just over 10 weeks before England's opening World Cup match on June 12 against United States.
Rooney broke a metatarsal bone in his foot in the buildup to the 2006 World Cup and his lack of fitness was cited as one of the reasons for England's quarterfinal exit.
But United's immediate concern is whether Rooney, who has hit 34 goals for United and one for England this season, will recover in time for Saturday's Premier League meeting with title rival Chelsea.
Rooney may also be a doubt for next week's return meeting with Bayern.