Jose Mourinho was humiliated on his return to Chelsea as his former club blew away his Manchester United side at Stamford Bridge.
The Blues, who sacked Mourinho for a second time last year, led after just 30 seconds when Pedro capitalised on slack defending to roll in.
Gary Cahill smashed in the second after United allowed Eden Hazard's corner to bounce in their box.
United offered little sign of making a comeback, falling further behind when Hazard drilled in a precise 15-yard strike.
N'Golo Kante skipped around a static defence to slot in and seal victory as Chelsea moved within a point of Premier League leaders Manchester City.
The comprehensive win lifted the Blues above Tottenham into fourth, with just one point separating the top five.
United stay seventh as the gap between them and the early pacesetters widens to five points, with almost a quarter of the season gone.