The England striker was initially ruled out for "two to three weeks" by Sir Alex Ferguson after injuring his knee in training on Christmas Day.
But Ferguson now expects to be without Rooney for a further fortnight, ruling him out of the meeting with Liverpool at Old Trafford on 13 January which can be seen live on Sky Sports.
He said: "Wayne is still injured. It is not serious but it is going to take longer than we thought, probably another two weeks.
"He won't be fit for Liverpool. Hopefully another couple of weeks will sort it out. Nonetheless, it is a loss. You always want your best players available."
United have scored 10 goals in three games since Rooney was injured, three of them going to his direct replacement Javier Hernandez.