It was not as if Kevin De Bruyne went under the radar. The Manchester City midfielder set up his team’s first four goals in the 6-2 FA Cup win at Luton on Tuesday night. He was sensational. It was just that Erling Haaland was better, even more eye-catching. The centre-forward applied the finishes to those four De Bruyne assists and banged in a fifth goal before the hour to ensure that he dominated the post-match discussion.

The thrust of it was that after almost two months out from early December with a foot problem, Haaland was back in the groove, at peak fitness. Pep Guardiola said he knew it would take time. For a player with such a hulking frame, it does. And when Haaland talked of feeling “amazing”, it added to the ominous vibe. A penny for the thoughts of Manchester United’s central defenders, who must steel themselves for the fight against him in Sunday’s Premier League derby at the Etihad Stadium.

Related: Erling Haaland’s attack on history points to new way for Manchester City | Jonathan Liew

But if Haaland could not have done what he did without De Bruyne, there was also a symmetry in terms of the latter’s injury comeback. De Bruyne’s story is more dramatic, it reaches further back and the way he talked, the fallout remains ongoing. What ought to concern United and the rest of City’s upcoming opponents is that De Bruyne has surprised himself with the manner in which he has returned. “I did not worry but I did not expect to come back in this way, if I am being honest,” he said.

De Bruyne has 11 assists (and two goals) in 10 appearances since his comeback from the hamstring injury sustained in the opening game of the league season at Burnley, which needed surgery and a five-month layoff. It was the same muscle that went in last season’s Champions League final against Internazionale, forcing him out of City’s treble-clinching glory after 36 minutes.

It has been a frustrating time, when De Bruyne has needed to tread carefully even after his comeback against Huddersfield in the FA Cup in early January. Guardiola introduced him gradually – as a substitute for three games, then mainly as a starter, but left him on the bench in the home win over Brentford on Tuesday of last week. He would play him only as a late substitute in Saturday’s victory at Bournemouth.

“I feel good,” De Bruyne said. “I think only against Brentford I was a little bit tired and I had a kick on my other hamstring. I just wanted to make sure that I didn’t risk that but I feel fine and I hope to maintain that.”

De Bruyne was asked whether it was about knowing his own body, with the inference being that he maybe knows it a little better now. “Yes and no,” he replied. “Normally I just play on but that’s mostly been my downfall before. In a way, I try to take care of myself but I also make decisions. Since I came back, my hamstring from the surgery has been great so I’m very happy about that. But I know if you don’t play for five months and come back you’re going to get niggles everywhere else. It’s normal and it’s fine.

“A game like Luton helps. I feel like I came back in a good way after being out that long. I could not ask for anything more because after five months you don’t know what could happen but I feel like I came back in the right way. I am pleased about that.”

Inspired by De Bruyne and Haaland, City were frightening against Luton, sweeping into the FA Cup quarter-finals – which has long been a key staging point for Guardiola. Only once in eight seasons have his team fallen short of this stage of the competition.

It was a night when it felt natural to wonder whether City could pull off another treble. They have normalised the abnormal. They sit second in the league, one point behind the leaders, Liverpool, who they face at Anfield on Sunday week. They have Arsenal at home on 31 March. In the Champions League, meanwhile, they are 3-1 up after the away leg of their last-16 tie against Copenhagen. The return is next Wednesday. De Bruyne, though, did not want to offer any headlines on this subject. He had given enough.

“We are in all the competitions that we need to be in and we are fighting for them,” he said. “That is something that is really positive after the seasons we have had before and going far. You hear about other teams, sometimes there is a drop-off, but if we win or don’t win we are fighting until the end so the drop-off has not been that much.

“Most of the big games come in March for us and if you are close to the top after March you basically fight to the end in the games that are coming. We try and get through that as much as possible and see where we are.”