Exactly why the team should be built around him in midfield, not in defence to accomodate Wanyama.
It should be Wanyama in defence if Pocchettino is determined to start both, as he's more limited with his passing and seems to stifle Dembele somewhat.
If I was Dier I'd have a word with the boss and tell him that he sees himself as a midfielder, here or elsewhere. Franco Baresi found himself in a similar position with the Italy national team, being played in midfield to accomodate Scirea as sweeper, then refused to compromise and opted to sit out instead. Great players have to play in their best positions, and Dier can become the best in Europe in his role.
Being versatile only benefits limited footballers like Milner, Phil Neville and John O'Shea; Dier is too good to be a utility player.