He was very flawed with the ball at his feet, had a number of bad turnovers and eventually just resorted to giving everything to Toby to deal with, which also put Toby in a terrible spot because Dier was not an option for Toby to move the ball to if he had nothing.
It is actually a very telling game as to why we need to move on from Dier. I agree with most here that defensively he was very good to great. I would say this is about as good a game as we could hope from Dier, and because of how poor he was on the ball it wasn't good enough and shows why he will never be what we need.
I give him full marks for the effort and defensive shift he put in, but he clearly needs to be sold in the summer. Should be near the top of the list of guys to go.