I think they already have a replacement for Fernandinho in Claudio Gomes the lad is only 18 but from what I have read City do have a real gem in the making if he fulfills his potential. He has been described as a blend of Makelele and Kante. Well lets see how that puns out.
CDM is a position that requires a lot of maturity, discipline, experience and reading of the game. I don't know of any CDMs being first team players before the age of 21-22, so even if Claudio Gomes is the brightest of stars it's unlikely that he'll be trusted to be a first team regular in that position for another 3-4 years. With no guarantee he'll end up good enough for the first team either.
They can't wait for another 3-4 years and play Fernandinho twice a week till he's 36-37. They need a mature and ready 26-27 year old CDM who will take over and be 30 by the time Gomes starts to hit his stride and challenge him for minutes.