De Gea isn't suited for a high line because he doesn't sweep, doesn't control his box and retreats to his line when he's facing 1-on-1s. Maguire isn't suited for a high line because he's too slow on the turn and most attackers can do him for pace. Lindelof isn't suited for a high line because he tends to lose his one-on-one physical battles. Early days but Martínez seems to be struggling with those physical battles as well. Varane should theoretically be alright, but he's a way off his best, and Bailly is just too unreliable. Dalot, Telles and Shaw are all suspect defensively. AWB has other defensive strengths to make up for his lack positional sense but lacks composure on the ball. Of all those players, only Maguire, Martínez and maybe Lindelof and Shaw at a push are any good on the ball.
Going into the season I think our best option all round if we wanted to play a passing game with a high line was AWB, Maguire, Varane and Shaw in front of Henderson with Martínez in either as either a 3rd centreback or as a holding midfielder. feck knows what we do now Henderson is at Forest.