Mourinho's tinkerings meant a first start of the season for midfielder Scott McTominay, playing on the right of a three-man defence, while Anthony Martial played instead of the dropped Alexis Sanchez. On this evidence, more changes may be needed when United take on Valencia in the Champions League on Tuesday.
'It's only 13 games in, that's the most important thing and there's so many points to be won. You need to carry on for 38 games and show consistency'
A deflected strike by Pedro Obiang in the 72nd minute and a goal from Andre Ayew five minutes later, when he stabbed home a loose ball from close range after a corner, were enough to end Huddersfield's unbeaten start to the season.
'I co-operated excellently with him at Chelsea, and the fact that Mourinho is the coach at Manchester was the decisive factor in my choosing my new environment'
League One Burton, managed by Nigel Clough, the son of former Nottingham Forest boss Brian, must endure another 90 minutes at their own ground on Jan. 23 before City formally book their place in the final.
The game at Huddersfield is the first meeting between the clubs since 1972 and the Yorkshire side have not won one since 20 years before that, in March 1952.
An ecstatic home crowd at a noisy and rainy John Smith's Stadium celebrated what was the Yorkshire club's first win over United since 1952 and one which few would doubt they deserved after a passionate display.
Here are five talking points from the weekend's action:
Talking points from this weekend's Premier League action as top two Manchester City and Liverpool both won with City remaining a point clear with two games remaining.
IMAGES from the EPL matches played on Wednesday
City will look to extend lead against United at Old Trafford
News of all that's transpired on and off the football field
IMAGES from the English Premier League matches played on Tuesday
Following is a match by match look at the English Premier League's weekend fixtures:
Images from the English Premier League matches played on Tuesday
Cardiff had lost six of their previous seven games but, knowing that anything but a victory would probably end their survival hopes after one season in the top flight, overcame a strong Brighton start to play with growing confidence and fully deserved their victory.
While few doubt Guardiola is capable of continuing his revolution at City, it is questionable whether his approach will have a broader influence on tactics and coaching across the Premier League.
Tottenham Hotspur striker Harry Kane was named player of the month after scoring six goals in four league games.
News of all that's transpired on and off the football field
Sam Allardyce has been named England manager on a two-year contract, the Football Association said in a statement on Friday.
News of all that's transpired on and off the football field
IMAGES from the English Premier League matches played on Wednesday
Images from the English Premier League matches played on Saturday
News of all that's transpired on and off the football field
Two big derbies and an eventful weekend of football awaits us all...
News of all that's transpired on and off the football field
Guardiola says City's points record may last years
Sanchez, 28, is currently in the final year of his contract. He has been linked with a move to Paris St Germain in the current transfer window.
Images from the English Premier League matches played on Wednesday
News of all that's transpired on and off the football field