Pep Guardiola will never regret for his decision to release some players in this summer, because his plan always bring good result at the end of the tournament, which I know that the players that left will not affect the Manchester city performance in this season.
Seems Manchester city aren't very active in the transfer market and that only means Pep Guardiola is confident in the players he has at the club currently. He trusts them because he believes they cab deliver him the champions league and the premier league because if he didn't trust them then he would had been very active in the market looking for players to add to the team to make up for those that have left the club but he isn't doing that. Pep Guardiola is a genius so he can use any player he wants to win trophies and if he noticed he can't achieve anything with this current squad, the January transfer window is open for Manchester city to go into scouting.
The strength of Manchester city isn't Haaland as he has been ghosting in major tournament that the clubs and fans needed him to win them the competition but there's no doubt that he has been very important in his team getting to to finals. Haaland need to work on himself more because this season won't be like last time, defenders will be keeping eyes on him more but this can also work in his favor as attention get withdrawn from him in big games since he hasn't showed any capability of causing damage in big games.
Considering the experience and tactical ability of Pep Guardiola and the current squad of Manchester City, I think the club wouldn't feel any significant effect about the players that's left the club this summer transfer window.
IIkay Gundogan after his departure was immidiately replaced by Mateo Kovacic while Riyad Mahrez who's yet to be replaced wasn't a regular starter last season. The transfer window is still open which means that City can still sign few more players before the end of the transfer window. But even if they fail to make any more signing, City will still perform well in Europe