I will talk about 2 things when a club holds the status of defending champion, the first is in terms of pressure, certainly the club with the status of defending champion will be more under very high pressure, because we all know that winning will be easier than defending the title. And the second is in terms of confidence, of course, with their status as defending champions they are in very high confidence, because they have been able to show that they are able to become champions in the Champions League. The question is whether they will be in which point, the first or the second, if they cannot control the pressure, the title will certainly be taken away, and if they can make more confidence above the pressure they will be able to defend it.
Let me guess your question Sir, although it is too early to say this because we often see different champions in UCL. But I think they (Machester City) may will probably be in the second point, after capable to win trophy the Champions League maybe they can come back to good play again and even more solidly, the winning mentality is already within players of Manchester City. When playing in the Premier League they can also be consistent and can survive as champions again, it is possible they can do it again in UCL.
Looking at it from another angle and especially having Manchester City in mind will let you know that the feeling and assertion you made above might not be applicable to Manchester City. Manchester City is a club that has sought to be the champions of the Europe for so many seasons. They have been so good for all the seasons, yet they were unable to win the title.
Finally being able to win the title should give them the confirmation that they are able to do it and they will be able to do it again in the next season. So it should not be as you have described above. For almost three consecutive seasons, Manchester City has been the best team in the European Champions League, even when they did not lift the title.