He is a player who controls the rhythm of the game and at the same time a midfielder who always provides accurate passes to the striker so losing him must have a big impact on the attacks built by Manchester City.
De Bruyne in many seasons, is a most creative player in Premier League in stats with most assists in one season. He's very important for Manchester City's attacking system, without him they will have less creativeness in attacking from key passes to assists. It's really hard for Manchester City to replace De Bruyne but no player can win against ageing and soon De Bruyne will no longer be able to play for Manchester City regularly or he will leave the club or retire his player career.
The front line is quite crucial for Manchester City because they rely on ball possession so if the midfielder is not good enough at holding the ball then the tactics applied by Pep Guardiola will not work perfectly.
Selling Mahrez to Saudi Arabia two seasons ago, selling Julian Alavarez to Atletico Madrid and decreasing performance of Grealish affected Manchester City too much in this season. They no longer have a deep squad like previous seasons and unfortunately they have many player injuries in this season. Haaland is a traditional striker who needs support from his teammates in key passes and assists. Without these supports, Haaland will nearly become a harmless striker.