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Chelsea, who are ten points behind league leaders United, spent more than £70 million in the January window in order to breathe fresh life into their defence of the Premier League crown.
Although the deficit may appear insurmountable, Chelsea still have to play United home and away before the end of the season - and Rooney is wary of the threat they will offer.
"We certainly haven't written Chelsea off," he said. "They are a big, strong side and we know if anyone is capable of going on a good run then it is them. But we know it is up to us where the title goes and hopefully we can win it back."
Despite his disappointing scoring return this season, which is nowhere near the heights he reached last term, Rooney insists he is not concerned.
"I have been happy with my play but obviously I want to score," he said. "As a striker you want to score goals but I have been happy with my performances, apart from the Blackpool match last week when I just couldn't get back into the game [he was subbed for Hernandez].
"I am delighted to have scored and hopefully I can go on a run now and help the team to win games."