I think a lot of the comments in here were equally levelled at Carrick when he came. I thought he was decent, but not that good and that the fee we paid was ridiculous.
I guess it's just not a glamour position, but when you consider what you want a DM to do, protect the back 4, break up attacks, set up attacks, then it is a massively important role for unsung heros.
Just have a look here:
https://www.redcafe.net/threads/utd-sign-carrick.121630/
Carrick is one of the most comfortable players on the ball I have ever seen, very like Modric. He didn't just start attack with a short or long ball he held the ball up and pulled the strings, Dier doesn't have the vision or creativity of Dier.