United head into Monday's derby clash with a 15-point advantage over nearest rivals City with only seven games left in the league campaign, but Rooney is refusing to take anything for granted.
A late slump allowed City to snatch the title from United's grasp last season and Rooney fears that one defeat could lead to another dismal run of form.
"One bad game and everything could fall and so we go out and play each game with a positive attitude," Rooney told the Daily Mail.
"We have to stay focused. We have to make sure we keep doing our job and keep our concentration."
Rooney hopes to be passed fit after a groin injury and wants to erase memories of City's title triumph when Sergio Aguero netted a last-gasp winner against QPR to break United's hearts.
"To lose the league in the last second (last season) was a bad feeling," said the striker.
"But when you are at a club like United, you have to be full of motivation and desire to win every year.
"That comes from the manager and all the players are aware of it.
"The club and the players have to be full of that desire. That's why the club has been so successful."
Rooney could line up in attack alongside Robin van Persie, who was linked with a move to Manchester City last summer but instead went to Old Trafford and has played a key role in their title charge.
And Sir Alex Ferguson has insisted ahead of the derby that the Dutchman never came close to joining City.
"It's hypothetical, he was never going to City," said Ferguson.
"He's been fantastic for us, a really good footballer. We needed a bit of maturity up front and he's been terrific.
"The improvement is not just through Robin but even when he doesn't score he's a fantastic footballer."