The decline in Haaland's performance is due to several causes and it is impossible for a quality player to have decline in performance without there being cause and I don't know what the cause is for sure
The reason for Haaland's decline in performance is just because of De Bruyne's absence. Last season, when de Bruyne was active in Manchester City, he did create most of the opportunities that Haaland was always having, and we all know de Bruyne to be a talented player, he is always creating opportunities for Haaland, and we all know that Haaland is good at positioning himself and scoring goals. But since de Bruyne has been absent since the beginning of the season, Haaland performance has also dropped, he has not been scoring the way he was scoring last season, and nobody is giving Haaland passes the way de Bruyne do.
Indeed, debruyne has been collecting lots of assists last season and missing him is greatly affecting the performance from city but it's not only that. City has lost also a player that was consistently scoring too. De bruyne was always consistently scoring a lot as attacking midfielders in any season and missing him was a disaster for manchester city. It feels like city lost its souls since de bruyne has always become the key player that was always organizing the attack from manchester city.
Losing him likes losing someone who who can distribute the idea that out of the box to its opponent. De bruyne has become the main partner of haaland since these guys have very strong chemistry right now.
Pep needs to find his replacement or they will be losing a lot. Manchester city is still able making its comeback again as long as pep will able to get the right partner for haaland that can perform as good as de bruyne.
This is why manchester city has been projecting some names to be recruited by manchester city to be a successor for him. I hope that all of plans will be going so well for pep guardiola.