United trail City by a point with the Manchester derby three weeks away and they will need to make a vast improvement if they are to keep pace with the champions.
Anthony Pilkington's header was enough to earn Norwich a famous victory and condemn United, who were without the injured Wayne Rooney, to their third defeat of the season. While United have come from behind to win eight times this season, they were unable to muster a response against Chris Hughton's in-form side.
"They deserved it because they worked so hard for it," Ferguson said. "We had a lot of possession and one or two half-chances without having any great chances. It just wasn't our night."
Chelsea were another of the frontrunners to suffer defeat. At The Hawthorns, Peter Odemwingie made it a miserable return to the ground for Roberto Di Matteo, as the former West Brom manager saw his new side lose 2-1 and sustain a first away defeat of the season.
Eden Hazard had equalised Shane Long's early opener before half-time but Odemwingie's second-half winner sent Chelsea tumbling to a fourth straight league game without a win.
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