Here, Press Association Sport looks at five things we learned from City's 2-1 victory.
1
Guardiola's City look like major title contenders - even without Sergio Aguero
The loss of Aguero to suspension for this match looked to be a huge blow for City, but they coped more than admirably at Old Trafford
Particularly in the first half, the Blues produced more of the slick football that their new manager has introduced as they maintained their 100 per cent start to the season and brought United's winning streak to an end
2
Guardiola might regret replacing Joe Hart with Claudio Bravo
There was plenty of interest in how Bravo would fare here on his City debut after Guardiola's much-debated decision to bring him in for Hart - and the former Barcelona goalkeeper had a poor day to say the least
He is supposedly excellent with his feet, but there was little sign of that here
He made numerous mistakes, both on that front and in other ways, with one moment seeing him flap at a free-kick, allowing Zlatan Ibrahimovic to score
He was also perhaps fortunate not to give away a penalty
City fans, certainly, will be looking for a more convincing showing from the Chilean in future
3
Marcus Rashford looks more than ever like he should be in the United starting line-up
The teenage forward is yet to make the first XI under Mourinho, but the clamour for that to change - boosted by his match-winning cameo against Hull and subsequent hat-trick for England Under-21s - will only have strengthened after this performance
Introduced at half-time, he made an immediate impact and, while ultimately unable to salvage anything for his team, delivered another lively display overall
4
Henrikh Mkhitaryan may need time yet to get to grips with English football
Rashford came on for Mkhitaryan, who comprehensively failed to make the most of the opportunity he had been presented in the shape of his first United start
The Armenian summer signing is unquestionably a gifted player, but was unable to show that on the big stage here and his performance would suggest it will still be a while before he really starts making a positive mark for his new club
5
Tensions between Guardiola and Mourinho appear to be easing?
There had been plenty of focus on the relationship between the two managers in the build-up to this, their first head-to-head contest in English football
But both men had tried to play down their personal rivalry at their pre-match press conferences - and that was in line with what followed on Saturday, as they engaged in warm-looking embraces both immediately before the game and after the final whistle
Source : PA
Source: PA