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.
Manchester city moved quickly after learning that several of their players would leave this summer and they immediately looked for suitable replacements, one of which was Kovacic. I think the departure of Riyad Mahrez to the Saudi league team left the right winger position looking vacant and actually Phil Foden could fill it. But still, Pep Guardiola has to find new players to cover his main players and he has to find that player soon. Meanwhile, Manchester City's defense is also in negotiations with Josko Gvardiol from Leipzig to help strengthen the defense in next season's Champions League.
Apart from that I don't think there is any recent news and maybe an unexpected problem is when Pep Guardiola doesn't update his strategy because all the strong teams in the champions league must have found a gap in his flagship 3-2-4-1 formation and it is worth waiting to see if Pep Guardiola will give something new later or stick with the same strategy as the previous season to defend their title.