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
The Manchester City squad has been perfectly formed based on the success achieved last season, the departure of Ikay Gundogan will not have a major impact on Manchester City performance because there are still 10 other players who are very integrated with the philosophy used by Guardiola. Riyad Mahrez departure is not too worrying, he is not the first choice in Guardiola's squad, although Manchester City are not looking for a replacement for Riyad Mahrez it will not affect Manchester City appearance this season.
Guardiola will try to bring Manchester City back to the highest point in the UCL this season, his experience in managing strategies when busy schedules are needed to prevent his players from getting injured or exhausted due to being forced to play too often. Manchester City's potential in the UCL this season is still very large, they must support each other to advance to the final round, Guardiola tactics have also greatly supported Manchester City journey this season.