Wayne Rooney was stretchered off in the closing minutes at Old Trafford with blood pouring from a deep thigh gash sustained when the United star collided with Hugo Rodallega's boot as he tried to a block a shot from the Fulham striker.
United went on to win the game 3-2.
Television pictures showed a severe cut down Rooney's right leg and United manager Sir Alex Ferguson confirmed the injury was serious enough to keep him out for at least a month.
"It was a very bad one. He has gone to hospital. It looks like he will be out for four weeks," Ferguson said.
Unless Rooney makes a quicker than expected recovery, the forward will miss United's Premier League games against Southampton, Wigan and Liverpool, as well as the club's opening Champions League group fixture.
The injury is also a blow for England manager Roy Hodgson, who will have to plan without Rooney for his side's opening two World Cup qualifiers against Moldova and Ukraine next month.
"You have to be wary of infection and that kind of thing so he has gone to the best place and hopefully he will be okay," Ferguson added.
"Rodallega followed through and caught him, but it was an accident of course." – Sapa-AFP.