Amad Diallo was the star of the show in Sunday's Manchester derby, and it was his hunger and desire that made the difference in the end.

Even before United's amazing late comeback to win at Etihad Stadium, Diallo had been the bright spark for the entire game.

He was the one player on the pitch who was playing with freedom and fearlessness, and also the only one from either side who seemed to be enjoying himself and trying to make an impact whenever he got on the ball.

There was so much safe play out there from both teams, but Diallo was different and he eventually got his reward for his endeavour, first by winning the penalty that led to United's equaliser which changed the momentum of the game, and then scoring their winner a few seconds later.

It was Bruno Fernandes who scored from the spot to make it 1-1 but that situation only came about because Diallo was alert enough to anticipate Matheus Nunes' poor back-pass, and then stayed calm when he got inside the City box.

He didn't rush, which led to Nunes making a ridiculous challenge on him to concede a penalty, and that composure and confidence was typical of his whole performance because he seemed determined to impress his manager.

You could say the same for his second goal. Again, City should have done much better defensively because no-one was pressing Lisandro Martinez when he played him in, and Josko Gvardiol was much too high with his defensive line.

That left a big gap for Diallo to run into, and Nunes did not track him, but it still needed an unbelievable finish to squeeze the ball past Ederson and past Gvardiol, who made a complete mess of his attempt to backheel it off the line.

City might feel a bit unlucky to have been beaten, given they were ahead until the 88th minute, but they brought this result on themselves.

It feels like every mistake they make is costing them at the moment but I am not talking about errors when I say this defeat was self-inflicted, more that their approach allowed United to grow into the game.

You rarely see City play with as much caution as they did on Sunday, but it was more understandable for United to initially sit in and give them respect the way they did.

Amorim's side fell behind to another set-play, something they are really struggling with at the moment but, as the second half wore on, you increasingly felt that they could get something out of the game.

They dominated possession for long periods, grew in confidence, and started getting chances. Amad got in a couple of times down the right and also went close with a header, while Fernandes put a great opportunity wide.

By that stage, the nerves and lack of belief from City was so evident. I have never seen a Pep Guardiola side sit off a team so much, and I don't think it is an energy or fitness issue - it's more about confidence.

They needed to get higher up the pitch and take more risks to try to kill the game off but it felt like they did not believe in themselves to do that.

Even some of their play out from the back, with them going long more often than I can remember, showed that. They were not willing to play out and risk losing the ball because they had a narrow lead.

Even the best players on the planet can suffer from a lack of confidence sometimes, but it is so surprising to see it from the whole City team.

When they got thumped at home by Tottenham a few weeks ago, they at least tried to get back in the game and had a go at Spurs, but got undone on the counter.

This time, they offered next to nothing in attack after half-time.

Yes, they were ahead against United and I can understand teams protecting a lead sometimes and being a bit more pragmatic.

City might also have been thinking about what happened against Feyenoord, when they collapsed late on.

That negativity can stay with you, but not to this level.

They completely gave up possession for most of the second half, and this was not even against a good United side.

This was a United side who, apart from Diallo, were playing to contain City and for two thirds of the game did not have much penetration.

Yet City did not kill them off at 1-0 and, even worse, they did not even really get close to doing so.

That lack of creativity is a big problem for City at the moment. I know conceding goals is an issue for them as well but this is the first time under Pep that they are not looking like they are going to score either.

So, he has got two areas to address, and it looks like it is a huge task for him, especially because he has never had to deal with it before.

In contrast, this result is a huge boost for Amorim, and not just because it will get the fans onside and get the players to believe in his methods.

The biggest thing is that, especially with Diallo being the star of the show, it justified his treatment of Marcus Rashford and Alejandro Garnacho.

Whether you agree with his decision to leave them out of the squad for Sunday's game or not, his explanation sets the tone for the rest of the United squad.

It tells them that it does not matter who you are, if you are not at it in training, playing well enough or conducting yourself in the right way around the place, you will not be in his squad.

That sends a huge message to the players about repercussions for poor performances, that I don't think his predecessor, Erik ten Hag, did often enough.

So, in every way, Sunday was a brilliant day for Amorim. It will be remembered as Diallo's derby, but it could be even more significant for United's manager going forward.

Danny Murphy was speaking to BBC Sport's Chris Bevan.