Rooney sustained a minor knee injury in training on Christmas Day and Sir Alex Ferguson initially expected the striker to be back in a fortnight.
However, it now seems he will be missing for longer - and will sit out next week's clash with Liverpool.
"Wayne is still injured," said Ferguson.
"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."
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- Sir Alex Ferguson