Sir Alex Ferguson has declared that victory over Manchester City would represent one of his greatest results in his long reign at Old Trafford.
United go to the Etihad Stadium looking for a victory that would send them six points clear at the top of the Premier League and for revenge after City did a derby double over them last season to claim the title.
Roberto Mancini's men won 6-1 at Old Trafford and then 1-0 at the Etihad Stadium in April on their way to becoming champions of England for the first time in 44 years.
And Ferguson, who has won 12 league titles and two Champions Leagues, believes an away win on Sunday would rank among the finest days of his 26-year time in charge.
"If we win it will be one of our best ever results," Ferguson told MUTV.
The Scot has is wary of City's size, particularly because of his own team's defensive frailties. United have conceded 21 league goals this season, almost twice as many as City, and let in two in their 4-3 win at Reading on Saturday.
"[City] are a really good, powerful team with massive players," he added. "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."
There is a seven-point gap between City and third-placed Chelsea, who are 10 points behind United, but Ferguson is unwilling to rule others out of the title race.
"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."