Spot on.
Just because Wanyama has been better in central midfield and Dier was goof last year doesn't mean we have to shoe-horn him into the team. Depth and competition is crucial for success. Rose and Walker have clearly upped their games since Trippier and Davies have signed. Dier and Wanyama should be competing for central midfield and he should stay well clear away from centre back unless it's last resort and three of our four actual centre halfs (Vert, Toby, CCV, Wimmer) are all injured.
Wanyama clearly stifles Dembele, so his 'good' performances have been counter-productive. Yes he makes a lot of tackles and runs hard, but Dier naturally reads the game better and more importantly plays the ball forward and quicker. To neutralise Wanyama's primitive passing game, he should only be paired with Winks, with Dier-Dembele the alternative.
Dier would be perfect for United, and if Pocchettino doesn't give him a fair crack in the midfield he might be tempted by any one of the elite clubs that would take him with open arms. It isn't hyperbole to suggest he could become the very best in his position within a couple of years, and it's important that he isnt wasted in a utility capacity.