Not counting this season's new players, there have been 12 midfielders signed post-Ferguson, I think. Someone correct me if I'm wrong. I'm including Mata as he was at his best playing centrally. I'd rank them all as follows:
1. Bruno
2. Mata
3. Herrera
4. Pogba
5. Matic
6. Fred
7. Casemiro (sample size)
8. Fellaini
9. Schweinsteiger
10. Eriksen
11. Schneiderlin
12. van de Beek
Echoing a lot of what MadDogg is saying above: Schweinsteiger did pretty much nothing for us and barely even got on the pitch, no way I'd rank him above Eriksen who was very good for half of last season. Casemiro, even with his crap start to this season and dodgy form before that, has already been a much better player and done more for us in a season than Matic did in five. And Mata not only doesn't belong in this list but is also way too high - he was never actually at his best playing centrally, and there was a reason every coach in succession routinely picked other players ahead of him and played him out wide where his complete inability to physically compete with defenders was less of an issue.
Fernandes, Herrera, Pogba, Casemiro and Fred should be a pretty non-controversial top five. And as usual, I had to think for five seconds when I saw Schneiderlin's name before I remembered who the hell that even was. What a waster.
The problem was we kept playing him deep because he was a big lanky bastard, when all his skills were suited to the AM position
I've seen this a lot throughout Pogba's time with us and never really thought it's true.
He was never an AM the way Fernandes, De Bruyne, Odegaard etc. are, and it showed when we tried to play him there (e.g. when Mourinho used him as a number ten in a 4-2-3-1 every now and then, usually away to better teams). He always struggles to find space between the opposition lines and receive the ball facing his own goal in crowded areas - too easy to nullify and mark out of the game.
Whereas he was actually very, very good at picking the ball up in deeper positions and driving forward with it, dribbling past players or picking out passes. It's why nearly all of his best spells for us came in a 4-3-3 shape where he could roam around in the left half-spaces.