He excels at the defensive side of the game, breaks up play, reads the game well, protects the centre backs really well and finds himself in the correct positions defensively, covers a lot of ground.
The problem is that for a team at the level we aspire to we need more from him on the ball. We need some one who is competent in the first phase, this is how you build your attacks (think Busquets, Alonso, Fernandinho at City, Modric/Kroos, Pirlo, Marchisio). Schneiderlin can't do this and goes as far as to hide when we are trying to build attacks because he doesn't want the ball so we really struggle when he is on the pitch going forward.
Our problem is we don't have one player who can do both sides of the dm job, all can either do one or the other so playing them you are sacrificing something. Mourinho is happy to play Felliani, he is tae so is also a threat on set pieces but he doesn't excel as much defensively as Schneiderlin and can't build attacks in the first phase.
Carrick can build attacks but as this point his legs are gone so opponents would walk through or midfield too easily.
Herrera can build attacks but he isn't defensive minded but he would be my choice if playing a midfield two he is mobil enough and technically competent to play in the first phase.
Blind can do both roles but he is just so slow, lacks agility and recovery pace that again he would be a liability in there.
A midfield 3 would make the most sense because it would get blind on with Herrera who can cover for him and would be we have two players competent in the first phase who can play passes through the lines and would also allow Pogba to have a clearer defined role as the number 8 and focus on that.