When Ruben Amorim oversaw his previous win over Manchester City – with his old club Sporting in the Champions League – he was expected to push the reigning Premier League champions into crisis. That was early November and it was City’s third defeat in a row.

Amorim repeated the trick here to boost his new project at Manchester United and bring Pep Guardiola to his knees. There seems to be no way out of the misery for the City coach, this is his eighth defeat in eleven games in all competitions, the decline of his all-conquering team is clear and extraordinary.

For a long time it looked as if City would secure a much-needed victory thanks to Josko Gvardiol’s first-half header. They had played modestly, but United were not at their best. The last third was a busy time. And in the end that wasn’t the case and they were able to welcome a result that Amorim will use as a catalyst.

It was Amad Diallo, the standout performer of Amorim’s young tenure, who made the difference. First, he won a penalty after City’s emergency defender Matheus Nunes missed a back pass pathetically short. Amad made a rash tackle from Nunes with a great stop-and-go move and Bruno Fernandes equalized from the penalty spot.

To United’s delight, there was more. Amad was alert to run for a long pass and his first touch was beautiful as he picked it up and passed it to the side of the advancing Ederson. The second wasn’t bad either, a volley from a tight angle that slipped over the line and Gvardiol couldn’t clear just in front of it.

City had won the league’s previous three derbies here – scoring 13 goals. On this occasion there was only fear and lingering disbelief. The insecurities are everywhere.

There was a strange atmosphere during the game, both teams fought, the storylines were unusual. Take Guardiola’s opening speech on Friday about the thought of losing the dressing room. Imagine hearing that at the end of October when City were unbeaten in all competitions. There were even details about Guardiola’s diet. In the evening he sticks to soup because his stomach is churning so much.

As far as United are concerned, the problems are profound and largely relate to how little time Amorim has on the training ground to drive a major overhaul of their style. He is aware of the scale of the challenge.

It was all about those 90 minutes and the players present, even if two absentees cast shadows. Amorim had excluded Marcus Rashford and Alejandro Garnacho from his squad, a decision he said was partly based on what he had seen of them around Carrington, which sounded ominous.

Amorim’s idea was to be solid; hence Noussair Mazraoui and Diogo Dalot as full-backs, Amad further forward as right number 10. Harry Maguire came into the middle of the back three. United sensed there were gaps behind City’s defensive line and Manuel Ugarte saved Amad in the 26th minute but he shot wide. The offside flag went up.

The City breakthrough had not been advertised and when it came it was a huge win for them. From United’s perspective, it was both disgusting and unsurprising that it came from a corner. They have regularly failed to defend properly this season.

City played briefly and there was luck when Kevin De Bruyne’s cross deflected off Amad and was perfect for the run of Gvardiol. Dalot didn’t do enough and Rasmus Højlund was sucked into the ball. Gvardiol had the header free.

Injuries were part of the story. Guardiola was without Manuel Akanji and Nathan Aké, while John Stones was fit enough only to return to the bench. With Rico Lewis suspended, the coach only had three fully operational and available defenders; Maybe seven in the squad isn’t enough? For this reason, Nunes ended up on the left side of defense. United, meanwhile, would lose Mason Mount after 12 minutes – a bitter blow for the hapless midfielder.

Controversy arose after Gvardiol’s goal. Kyle Walker went head-to-head with Højlund after he fouled the United striker. Walker would fall to the ground, a shameful attempt by the 93-year-old England international to send off his opponent. Both were fully booked.

Phil Foden had a shot wide of the near post in the 21st minute and the City midfielder went close again in first-half stoppage time after a lightning-quick shot, but it was no thriller.

United had rhythm and structure in their passing movements but needed to show more personality and penetration. City were just happy to have something to hold on to. They invited United in the second half and if it was strange to see them playing with so little momentum, perhaps they thought United couldn’t touch them.

Amad beat Ederson with a header and Fernandes had a huge chance in the 74th minute after being released by Hojlund. When his sunk finish went wide of the far post, that seemed to be it for United. Amad had other ideas.