Honestly, I was already warming up to the fact that Manchester City will be slightly weaker next season after hearing of his departure. Kevin Debruyne was already becoming a peripheral figure in that Manchester City team, they may actually be stronger without him provided that his replacement is fit & at least 70% as talented. He didn’t feature in quite an amount of games last season and even days he featured and wasn’t playing well city still coped.
They always have something up their sleeves. So expecting them to be weak at any time is like expecting a mountain to suddenly become a valley. I already foresaw Foden taking up the mantle from him & becoming more central to their plans. Let's see how the transfer window goes for every team.
IMO, Pep Guardiola actually has a choice of other players, for example Phil Foden who plays the role of replacing De Byurne. It is not uncommon for Foden to play impressively when De Byurne is still injured. Apart from that, Phil Foden is a player who is capable of playing wherever Pep rotates him, it's just that Kevin De Byurne's role is still very much needed even though he is no longer young. Plus, Pep has confirmed that Kevin De Bruyne will not leave the Etihad this summer, quoted by Fabrizio Ramano. Can't rule out the possibility that if De Byurne leaves, City could weaken as you say. However, the fact is that the City players were able to do their job very well when KDB was suffering from a long injury. Pep Guardiola's rotation for several of his players is working as it should. Even if one of the City players leaves, for example the player we are discussing. City will find the ideal replacement, or he will play with the available players. After all, City has very capable players in every line. I will quote another report from Fabrizio Romano, Pep said, we have the opportunity to change the squad, I do not rule out the option of choosing new players, but if someone leaves, Pep said that he still has the same squad. I tried to interpret it, if one of the City players left. Pep still has a strong squad, he can even bring in new arrivals.