Agree that he isn't and should never be used as, a defender. Davies, Wanyama and even Carter-Vickers would be more solid options in there. I feel sorry for him as he's been moved out of his best position to accomodate Wanyama, despite being the more complete player.
The same has happened to Rashford; Ibrahimovic was brought in and he's been shoehorned onto the wing, with his form, momentum and reputation naturally suffering in the process.
Pochettino has his favourites, what can you say? He'd play Kane, Dier, Wanyama even if they all had one leg.