Chelsea got off to the best possible start on Jose Mourinho's first return to Stamford Bridge since he was sacked by the Blues last year. Just 30 seconds into the game, Marcos Alonso played a diagonal ball forward to Pedro who rounded De Gea and slotted home to give Chelsea an early lead.
Manchester United almost equalized when Antonio Valencia found the head of Zlatan Ibrahimovic from the right, but the Swede could only guide his header just over the bar.
The Blues then went 2-0 up midway through the first half Cahill took advantage of a loose ball from a corner. The ball bounced up for the England defender whose half-volley deflected off Daily Blind and flew past De Gea.
United's best chances of the first half fell to Herrera and Lingard. The former shot from distance which was well saved by Courtois, but the Belgian keeper could only parry the ball into Lingard's path, only for the England man to see his shot saved again.
Jose Mourinho tried shaking things up at half-time by taking off the inneffective Marouane Fellaini for former Blue Juan Mata, but the changes didn't make any difference to the pattern of the match as Chelsea continued to dominate.
With around 30 minutes still to play, Hazard made it 3-0 with a trademark finish, cutting inside onto his right foot and placing the ball in the bottom left corner.
If Chelsea fans weren't in dreamland already then they were when N'Golo Kante made it 4-0, scoring his first ever Chelsea goal and just his second in English football. Pedro laid the ball off to the Frenchman who came forward, easily beating Smalling and neatly finishing past David De Gea.
Ibrahimovic and substitute Marcos Rojo both threatened to get a consolation goal as they forced Courtois into two good saves, but this victory was as comprehensive as they come for Conte's side.