Manchester City are favourites,but they can't win the Champions league this season,to me,they lost a little depth in their squad when Mahrez and Gundogan left the team.It is true that Gundogan was a very good player at City,and his presence in the team then was really helpful to City.He was responsible for scoring important goals,just the way Rodri is now in the current squad,thats how Gundogan was.However,they go again in the race to win the Champions league two times,but I don't see that as a possiblity because so many good teams are also eyeing that trophy.Barcelona,Bayern,Dortmund,and PSG and Arsenal are all hoping lift that trophy, therefore,it's really going to be a tough one.
Despite that Manchester City is still winning their matches in the Champions League and they are also at the top of the English Premier League table, it is however notable that Manchester City performance of last season exceeds her performance for this season. There is a very slight drop in the form of the players and that of the team in general. I know that this is as a result of the sale of their two key players in the midfield as you have mentioned.
In as much as I may agree with you that it will be difficult for Manchester city to win the Champions League consecutively. I also do not doubt the possibility of that happening. This is because so many big teams are not in their Prime.
They are merely doing well in their respective local leagues, but once it comes to the Champions League, I do not see them as the perfect team. Manchester City is still one of the best and if not the best and the most favourite to win the Champions League of this season.