To replace Yaya, I think we may need two players.
Pogba is going to be a star, but can he control a game like Yaya?
Koke and Pogba would put us on par with what we would have if we were to have Yaya (2013/14)
Just one may see us lacking, with Koke we miss the finishing and presance, with Pogba the ability to dictate a game and passing ability. (although he's not far off)
I'd sign both but it's going to cost about £130m for the two, doubt that could happen in FFP terms. Going to be a tricky and exciting summer to say the least.