Sir Alex Ferguson says victory against Manchester City on Sunday would be one of his side's "best-ever results".
Premier League leaders Manchester United travel to the Etihad Stadium three points clear of champions City, whose European adventure ended in midweek after finishing bottom of their UEFA Champions League group.
Ferguson also expressed concern over his fragile defence and challenged his side to learn from their error-strewn display at Reading last weekend.
"If we win it will be one of our best-ever results," Ferguson told MUTV.
"They are a really good, powerful team with massive players.
"It won't be easy and if we defend like we did at Reading we'll be in trouble.
"The important thing for us is to take lessons from the mistakes we're making and do something about it. We need to find a solution."
With Chelsea and Arsenal sliding off the pace in recent weeks, there have been suggestions it could turn out to be straight battle between the two Manchester clubs this season.
But Ferguson, who has a fitness doubt over Tom Cleverley, is not buying into that assessment.
"At this time in the season you can't really say it's a two-horse race," he said.
"If you think back we were 12 points behind Chelsea at one point and when Arsenal beat us in the league in 1998 we were 11 points clear."
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