The Premier League champions, who trail leaders Manchester City by two points, head to third-placed Spurs on Sunday thinking only of their own title challenge rather than the fact they can almost certainly destroy Tottenham's with a victory.
Their bid to maintain an excellent record against the north London side, whose last victory over United was in May 2001, will be boosted by the return of top scorer Rooney who missed their last two games through illness.
"We realise... how important it is on Sunday to maintain our challenge, we're not interested in anyone else's challenge, but to maintain our challenge we need to win on Sunday," Ferguson told a news conference. "It's the most important game for us so far now.
"If you look at the rest of the season, you would certainly say it's building up to what could turn out to be the decider with City [at Eastlands in April] but this is our hardest away game [before that], no doubt. Hopefully we can navigate it."
Defender Chris Smalling is doubtful after a clash of heads on England duty midweek, while midfielders Tom Cleverley and Antonio Valencia are ruled out with foot and hamstring injuries.
"Rooney's fit, he's trained all week which is good news," Ferguson said.
"Smalling - there's no concussion but it is quite a nasty cut, whether we can patch it up I'm not quite sure. It's something we'll need to tread carefully with. We'll see what he's like tomorrow."
Striker Michael Owen, who has been out for more than three months with a thigh injury, has returned to training.
"Hopefully the lad has a bit of luck, he's not had much at all," Ferguson said. "When he has an injury it's always a long-term one but hopefully he's back for the rest of the season which will help us."
'STRANGE GAME'
Ferguson singled out midfielder Michael Carrick's recent contributions as being key at an important part of the season and was hoping for more of the same from a player whose publicity-shyness was "refreshing".
"He always does better in the second half of the season," the Scot said.
"I know he disagrees with me on this but I feel the second half of the season sees him at his best and he's doing that at the moment, he's doing very well so it's pleasing."
United head to White Hart Lane to face a team still reeling from last weekend's 5-2 thumping by bitter rivals Arsenal which came after Spurs had established a two-goal lead.
The North London side are eight points behind United, who have 61 points from 26 games, and 10 adrift of City, and defeat by Ferguson's men would probably leave them with too much ground to make up for a realistic title tilt.
With Spurs on the rebound, Ferguson knows his side face a tough game and refused to read too much into the Arsenal defeat.
"It was a strange game, I think derby games can be like that, you can't predict them too much," said Ferguson, whose side had a derby nightmare of their own earl
