United have struggled badly during David Moyes' debut campaign as manager, suffering their eighth Premier League defeat of the campaign at Stoke last Saturday.
"We know it is going to be extremely difficult to win the title now," said Rooney.
"There are a lot of points in between ourselves and a few teams.
"Realistically, the top four has got to be our aim. Although, as a club, we always want to win the league title maybe it might be a bit too far."
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