Silva isn't the problem in the 433. Silva has an excellent work rate and can pass and move on the level of Barcelona's midfield, which is clearly the model Txiki, and by extension Pelle, want to follow. If it weren't for his recurring battles with foot injury, Silva could slot right into Barcelona's midfield, replacing Iniesta. A hard working midfield trio is the most important part in a 433, and that's where Yaya falls short. Whether he can or can't track back and defend isn't at issue. All that matters is he won't. We've seen that time and again. If we did get Pogba and De Bruyne, our best option is to play Yaya as an impact player off the bench, not in the starting XI (barring injury). I would put faith in a midfield trio of Silva, Fernandinho, and Pogba and play De Bruyne as part of the attacking triumvirate with Sterling and of course Aguero. Yaya would be an excellent impact sub off the bench, and would be important to rotate in to keep the squad fit, but the transition to 433, Yaya's age and declining fitness, and, most crucially, Yaya's lack of work rate when he's not going forward are all reasons to phase him out. Even if Pogba doesn't sign, it might be worth putting faith in Fernando, who has shown improvement, and gradually transition him to starter in a midfield trio of Silva, Fernando, and Fernandinho. Fernando's style of play suits a 433 more than 4231 anyway.
And of course this is assuming Silva remains fit throughout the season, which I don't see happening.