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
Manchester City is set to confront fresh challenges after winning the UEFA Champions League title; they are the current reigning Champions and widely regarded as the best club in the world. Pep Guardiola does not appear concerned about his departing players; perhaps he has a backup substitute for every key player at the club. Chelsea's Kovacic will take over for Ilkay Gundogan. Manchester City's performance would be smoother than ever before, thanks to Pep's relentless work on the club's anticipated performance, always ready to take things to the next level.
As long as the players substituting the departing ones are of the same league and they can find themselves harmonizing well with the team then it is only natural for Guardiola to not be concerned.It is very difficult to win the UEFA Champions League twice in a row so even for a team like Manchester City who could easily beat anyone last season it will not be the same and it will not be easy at all to repeat their glorious 2022/2023 season.The other teams though so far have not done much in the player transfers during the summer so I don't expect them to have an easy win in the Champions,mainly I am referring to Real and Barcelona here.