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.
Yes, your opinion is very correct, but for me, De Bruyne's injury is not a big problem for Haaland because since De Bruyne couldn't appear with Manchester City, Haaland performance is still very good, he has even scored 14 goals in 15 matches and that means Haaland seems to be doing well. without support from De Bruyne but in my opinion it looks like Haaland is a little tired and the injury he suffered has had a negative impact on Haaland performance.
Meanwhile, currently Manchester City is appearing without these two mainstay players and has experienced a very drastic decline, but Guardiola intelligence has managed to keep this club in a safe position and if Haaland returns, I'm sure he will be very quick to get first place in the points table.