The only hope for their title rivals, including a lethargic Tottenham side who were lucky to beat Crystal Palace, is that the fixture pile up in December halts the runaway leaders.
Some excellent substitutions from David Unsworth confirmed that Everton have been lacking pace all season, while West Ham's sloppy set-piece defending has sealed Slaven Bilic's fate on an eventful weekend of Premier League action.
Here are three tactical lessons we learnt:
1) Mourinho has finally accepted that he must attack his title rivals
Jose Mourinho's ultra-defensive tactics when facing title rivals usually work when he is in a division with just one or two challengers, but in the Premier League – where the "big six" means he has ten such games per season – this simply won't work. The Portuguese appears to have finally got the message.
Ashley Young and Antonio Valencia held very high starting positions, United used a front three, and the entire team pressed high throughout the opening 45 minutes – possibly to unsettle a Chelsea team who looked psychologically vulnerable during their 3-0 defeat to Roma in midweek. It made for an excellent half of football as United left gaps for Chelsea to counter-attack, and suggested that Mourinho has accepted that he must be braver in his tactical approach.
However, United dropped much deeper in the second half and paid the price. Chelsea scored the winning goal because they enjoyed increasing possession – and because United didn't bring on a third central midfielder in time.
2) Winks still needs to learn how to change the energy of a Premier League match
Tottenham Hotspur were far too lackadaisical on Sunday, stealing the points thanks to an individual moment of brilliance from Heung-Son Min on a day defined by low-tempo football and Crystal Palace's stubborn defending. Spurs were aimless in possession, rarely making runs in behind or working hard to create space, instead passing the ball slowly at the back – which allowed Palace to keep their shape.
Only five of Harry Winks' 51 passes were forward, highlighting how he was unable to change the stagnant atmosphere. Time and again he picked up the ball, saw a lack of movement ahead of him, and played an easy five-yard sideways pass, even though Palace's formation meant he was the only Spurs midfielder able to get on the ball.
Winks needs to learn how to lift his team-mates and set the tempo by playing with greater energy, rather than being led by the pace of those around him; the 21-year-old should have tried to dribble forward on the ball and cut long vertical passes to unsettle Roy Hodgson's team.
3) Lack of pace up front makes West Brom favourites to go down
Failing to score against ten men is always disappointing, but Tony Pulis deserves criticism for his side's inability to muster more than three shots on target in this match. He has built a team almost entirely comprising of stocky footballers, which has led to even stodgier football than usual this season; West Brom are averaging just 2.7 shots on target per match and have scored five goals from open play.
Hal Robson-Kanu clearly isn't good enough to play at this level but gets selected purely because there is no pace anywhere else in the side. Meanwhile a central midfield of Grzegorz Krychowiak, Gareth Barry, and Jake Livermore is never going to lead to chances. Between them, these three have scored a mere six top-flight goals in the last four seasons.
Best of the Week – David Unsworth's substitutions
Throughout the 2017/18 campaign Everton have been accused of being too leggy and Unsworth has sought to correct this problem by playing plenty of young, quick wingers. It finally paid off at Goodison Park on Sunday evening as his substitutes - Ademola Lookman, Dominic Calvert-Lewin, and Aaron Lennon – turned the match on its head.
Watford simply couldn't deal with the directness and energy of this new-look Everton attack. First Lookman drove forward in possession and played in Oumar Niasse for the first goal, then Calvert-Lewin nodded in the second as Everton continued to cause problems with their pace in wide positions. Finally Lennon's turn in the penalty area earned them the penalty for the winning goal.
Three perfect substitutions led directly to three Everton goals.
Worst of the Week – West Ham's naivety
West Ham were playing well when, in the 20th minute, they got over-excited and only left two players back for a corner. Up until that point their deep blockade had shown bite in the tackle and determined defending, suggesting they could produce the sort of under-dog performance that defined Slaven Bilic's first season in charge of the club.
But naïve defending ultimately cost them yet again. The Hammers left just two defenders to prevent the counter-attack from a corner and, more importantly, didn't position a single player on the edge of the box to pick up a loose ball. Sadio Mane and Mohamed Salah were gifted the opener, confirming to the West Ham board that Bilic does not have the tactical intelligence to halt their poor form.