As it stands right now, Chelsea are looking certs to win the league this season, and in recent seasons they have also won the Champions League and the Europa League. They have achieved this with a regular turnaround in managers, yet none of them has ever employed Yaya. The current manager, Mourinho, has achieved multiple successes wherever he has been, including the Champions League at two different clubs, yet at none of these clubs did he ever employ Yaya.
The point I am here making that a quality squad and a top line manager would never be so over reliant on one player, and if City DO sell Yaya, then it is up to the manager of the day to get on with it and not to seek excuses, and then to make his remaining or altered squad adapt to whatever formation that the loss of Yaya would bring. The game will go on long after Yaya and we should always be prepared for this occurrence, as we should with the inevitable departure of any of our players.