The striker was brought on in the second half as Sir Alex Ferguson's side secured a 3-2 win but was withdrawn in the closing stages with a nasty-looking cut to his right leg
The striker looks set to miss a considerable amount of playing time for both club and country, with England set to take on Macedonia and Ukraine in early September, while United face up against Southampton, Wigan and Liverpool within the next month.
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Sir Alex Ferguson confirmed after the game that Rooney has since been sent to hospital, and could face a month in rehabilitation.
"It's a very bad gash. He's been sent to hospital and I think he'll be out for four weeks," Sir Alex told Sky Sports.
"Rodellaga followed through after his shot. Obviously it was an accident."
Focusing on the game itself, Fulham took the lead after just three minutes when Damien Duff latched onto Bryan Ruiz's free-kick.
United responded strongly with three first-half goals from Robin van Persie, Shinji Kagawa and Rafael - the former two with their first for the club.
A mix-up in defence between David de Gea and Nemanja Vidic saw the centre-back knocking the ball into his own net with 25 minutes still to play, but the west London outfit could not find the all-important equaliser.