City's weak underbelly was again exposed, providing a template for the entire division in 2018/19, while United's resilience suggested Mourinho is finally rubbing off on these players. A genuine title challenge next season is definitely on the cards.
Elsewhere, a series of draws in the bottom half of the table left Southampton and Stoke City deeper in the relegation zone, while Chelsea's dropped points against West Ham United confirmed they will be playing Europa League football next year.
Here are three things we learnt from the weekend action:
1) United's victory removes Pep's psychological advantage, suggesting Man City will be fearlessly attacked in 2018/19
Having shipped six goals in 180 minutes of football last week Man City's stranglehold on English football has loosened. They will spend big this summer and recollect themselves, but one of the most important aspects of Guardiola's system is psychology; now that teams know to press high and attack their soft underbelly, City's supremacy can be called into question.
Mourinho's United are the primary candidates to challenge them at the top, not least because the Portuguese has finally begun to take a more confrontational approach in big games. The way they tried to stop City playing out from the back, and were competitive higher up the field for the first 20 minutes (even though this led to a period of City domination), suggests Mourinho is capable of adapting his managerial style.
As teams from all levels of the Premier League adopt an aggressive tactical approach to City games from now on, it is possible Guardiola's will drop more points – and become more nervous at the back – in 2018/19.
2) Hernandez has a big role to play for West Ham at the tip of a 3-4-2-1
David Moyes's 70th minute substitution completely changed the pattern of the game at Stamford Bridge; Chelsea's total dominance evaporated as West Ham United played their part in an open final 20. Javier Hernandez scored the equaliser within three minutes of coming on, suggesting that the Mexican's finishing could save them from the drop.
Despite playing a narrow 3-4-2-1, in which midfielders Joao Mario and Edimilson Fernandes started behind the striker, West Ham were extremely soft in the centre. Moyes seems incapable of patching up their most glaring tactical flaw even with four centre-mids on the pitch. On this occasion, Willian, Eden Hazard, and Alvaro Morata carved them open time and time again.
But when Hernandez was introduced (and Marko Arnautovic dropped deeper) suddenly the game became stretched. Chicharito's runs off the shoulder of the last defender, excellent hold-up play, and clinical finishing were a reminder that he is a top level striker – exactly what the Hammers need if they are to win this weekend's crucial clash with Stoke City.
3) Southampton discover a tactical system that can serve them well for the remaining six games
Mark Hughes's Southampton were beaten yet again on Sunday (they've won just once in the league, at West Brom, in the whole of 2018) but there is a glimmer of hope. A new formation, similar to the one deployed by West Ham, gave them a fluidity in attack we have rarely seen this season.
James Ward-Prowse and Dusan Tadic dropped wide to make a flat 5-4-1 during periods of Arsenal pressure, but when Saints went forward both players floated into central areas, linking together neatly with Shane Long – whose runs on the shoulder of the last defender provided Southampton with a quick counter-attacking outlet. Ward-Prowse's movement was particularly impressive, consistently giving Tadic an out ball and sucking Arsenal inwards, which in turn opened up pockets on the left for Ryan Bertrand.
Ultimately their defending was too weak to take a point away from the Emirates, but Hughes will be confident he has found a more stable tactic for their tricky run in. Their game against Chelsea this weekend is the perfect opportunity to build momentum; Antonio Conte's side tend to struggle against teams that mirror their 3-4-2-1.
Best of the Week – Morata & Hazard partnership
Chelsea should have been at least 3-0 up before West Ham's equalising goal on Sunday, largely thanks to the outstanding link-up play between Alvaro Morata and Eden Hazard. The former has struggled to adapt to English football, but at Stamford Bridge we finally saw the potential in his relationship with Chelsea's Belgian star.
Morata consistently dropped off the back three, finding room with his back to goal and playing neat one-twos with Hazard. They passed the ball 22 times between each other across the 90 minutes, tearing through the heart of the West Ham midfield despite Moyes packing it with bodies.
Worst of the Week – Allardyce's approach to the Merseyside derby
It's that stage of the season when certain matches become irrelevant. The Merseyside derby was one of the worst in years, partly because Jurgen Klopp rested so many key players before the second leg of the Champions League on Tuesday and partly because Sam Allardyce's tactics weren't set up to exploit Liverpool's bluntness.
This match was perhaps the final opportunity for Big Sam to show Everton fans he can be the expansive coach, or at least the winning coach, that they want. Instead, they amassed just one shot on target at Goodison Park – hardly a surprise given their cagey formation and the constant use of big target man Cenk Tosun. Allardyce surely won't last the summer.