You didn't look close enough, Dier spent the whole game moving backwards and forwards between midfield and defence, supporting both sections of the team with control and effectiveness.I don't agree with it.
I have to mention that I did not see the 1st half (I was on concert) but in 2nd half he really seemed off-steam. On 75th minute he gave ball away in midfield in very-very dangerous situation that nearly cost us a goal. And out of interest I started looking at him more closely - basically since then he did not put a foot right. Gave some stray passes or totally hopeless crosses over the top and even some short passes he gave to teammates near center circle were hard to control.
I think if Poch decided to go with Dier from the start he could have brought on Wanyama on 65th. Dier really has played a lot, and some fresh energy wouldn't have hurt.
The guy worked his socks off and was one of the reasons why we were able to pull back and stop them regaining control.