On the contrary, if City win the CL this year, than signing Messi is useless. They are the best in the world, doing what they are doing, why would you need Messi? City don't seem interested to sign very big names. They are invested into pleasing Pep for as much as possible. Pep, despite everything he might be saying, doesn't fancy Messi at this point imo.
I was thinking that and was going to post as such, that it would be just down to making their brand explode even more. How they would have to take a well functioning 11 and lose a bit of that to accommodate Messi.
But then I realised it is Messi we are talking about, who has 28 goals and 11 assists playing in a team who are either out of their prime or not reached their prime, with a manager who is levels below Pep, and a fitness level way below Citys. When I factored all that in, then sure you are going to lose a Bernardo Silva at False 9, but then you are getting a motivated Lionel "friggin" Messi surrounded by the fitness levels of NandroPeps. And for that, I think they will go up another level sporting wise too.