This thread is tiled wrong. The best mid to take Pogba's spot (if he leaves) is actually Pjanic. The more relevant question is who will be the addition to midfield to help bring the best out of Pjanic.
Pogba, in my opinion, is the best midfielder in the world in his position so there is no point trying to replace him. Pjanic was the 2nd best midfielder in Serie A and he is ours.