In what league? Maybe the CL, but when I look at the midfields of the top Prem sides I really don't see anything special. Fernandinho, Fernando, Eric Dier, Danny Drinkwater, Francis Coquelin etc were all in the teams the finished in the top 4 last season.
Ignore last season. Look at this season.
Fernandinho, Gundogan, Silva/kdb at City.
Xhaka, Cazorla and Ozil at Arsenal.
Spurs' overall team and work ethic is what makes it so functional (plus Dembele being a very good player in there). Take away Pochettino and Spurs overall would decline a lot, it's not really the players that make their team that good.
Liverpool play creative players all over the pitch, without strict positions for most of them.
At United, we are at our best when we play our most technical players like Herrera, Pogba, Carrick, Mata and soon to be Mkhitaryan I'm sure.
Schneiderlin isn't a patch on any of those and isn't good enough technically speaking nor does he ever take responsibility on the ball to do something. So if he isn't the type to be a pure holding midfielder and shield the back 4 really well, what the hell does he do? Like I said, at Southampton, he was not the defensive mid. That was Wanyama. He was used to harry the opposition and press players, provide extra solidity. Qualities that aren't needed at big clubs unless these players can actually do things on the ball. So until Schneiderlin shows he has ability on the ball and can pass it around well, he won't get a look in. Van Gaal bought him and then stopped using him because of how he was. Darmian has a similar mentality and never taking responsibility, and that's why neither of them are getting a look in under Mourinho.
Also, I don't understand why people keep going on about him not being at his Southampton's level here. Why would we want that? When you buy players from smaller clubs, you buy it in the hope that they can take the step up and raise their game to match new expectations, and improve on the qualities which we bought him for when surrounded by better players.