In Pellegrini's previously successful sides he played the 442/4222 with two proper defensive midfielders, not unlike how Southampton are set up with Wanyama/Schneiderlin. So I do not think he actually wants a like for like replacement for Yaya, who, even last season often left us exposed at the back when he was going forwards.
What I think he'll go for is a midfielder with the passing range to link the attack & defence, but who won't necessarily be making rampaging runs into the final third and who will be just as keen on defending. That would give us more solidity in European games as well if he continues with the 442.
Schneiderlin, Kondogbia, Witsel, Nainggolan.
Players who can pass well, and score goals, but whose first instinct is to sit further back and will make sure the defence is not exposed.
Whoever we get to replace Yaya though, we need to also buy an understudy. It's clear from the time we were without him this season that we don't have a viable option for when Yaya is missing - and if we simply replace Yaya with another single player, we'll still be over reliant on 1 player and are going to be shafted if they get injured for a long time. Maybe Zuculini could be that player, but from how his loan seems to be going, I doubt it.