Irreplaceable? Guardiola and Busquets disagree. It's not all about one player, the team come first. Maybe a like for like replacement isn't available, Pogba is probably the closest but then again Barcelona, Real Madrid and Bayern Munich don't have either and they're not too shabby. A good team will always beat 11 individuals of similar quality, Yaya leaving and a different type of midfielder may get the best out of our other players.
Just imagine how good Aguero could be with runners from midfield getting into the box and creating space for him instead of being left isolated because we have to play two defensive midfielders to allow us to accommodate Yaya's lack of energy in big games. That's not mentioning how much it could improve Silva and De Bruyne's option if they had midfield runners. Just saying.
Yaya is a ridiculously good player at what he does but I think as a team we suffer because of the lack of energy and movement in midfield. Yaya isn't responsible for that, Pellegrini is for continuing to select him and not replacing him but we will move on from him at some stage and improve as a team I believe.