According to The Mirror, Rooney left the club's Carrington training ground this morning and needed 10 stitches in his head after colliding with Phil Jones during a practice game.
He will definitely miss United's crucial trip to Anfield tomorrow and is likely to be ruled out for the England games on September 6 and September 10.
The Red Devils have already contacted the FA about the player's injury and informed them that it is highly doubtful that he will be fit for Friday's match.
The news comes as a blow to United, especially after Rooney's promising return from injury against Chelsea despite continued transfer speculation surrounding his future.
United take on Liverpool at 1.30pm tomorrow afternoon.