I rarely agree with the Nevilles but throughout last season they were spot on on two things.
a- we lack width.
b- time and time again United had allowed a big gap to be created between our midfield and attack which is being exploited.
Point A will be tackled by Perisic and since its not a Perisic thread I won't explore this point further
Point B is the result of us playing anyone with a vague idea about tackling as a DM. Fellaini and Herrera are not DMs and they will never acquire the discipline, the positioning and the vision that a top quality DM need.
Dier isn't perfect. He's not as talented as Carrick is and he's not as savvy defensively as Ince was (I'm avoiding Keane since he's a freak of nature, the wolverine of football). However he's a talented DM and he's very versatile. His experience as a defender AND as a midfielder puts him in a position of understanding both roles. If Lindelof move forward with the ball, then Dier know he will have to stay back to cover for him. If Herrera and Pogba need to be more advanterous because we need a goal, then Dier knows better not to join them. If United are caught in a quick counter, he would disrupt the opponents game without committing himself too much, up until backup comes. That's something that doesn't come natural for a no 10 such as Fellaini or a box to box player like Herrera
We've been spoiled by a freak of nature (ie Keane) who was followed by a lesser freak of nature (Carrick) whose our master man (SAF) carefully built a team around. There's no news Keanes or Carricks available.