After Zlatan Ibrahimovic had hooked a volley well wide for the hosts, Islam Slimani almost found Riyad Mahrez with a chested pass inside the area before Daley Blind scrambled away a dangerous ball from Christian Fuchs.
Zlatan Ibrahimovic chested down a lovely Pogba pass but blazed over the top, Marcus Rashford lashed wide after an Ibrahimovic touch and Pogba, finding his best form, saw a curling long-range strike pushed away by Zieler.
Three minutes later, Rashford prodded home Mata's low ball across the six-yard box to make it 3-0, and the rampant home side, pouring forward at every opportunity, made it four when Leicester's defence collapsed at a corner and allowed Pogba to thunder home a free header.
Leicester manager Claudio Ranieri rang the changes at the break, bringing on Andy King and Gray for star men Jamie Vardy and Riyad Mahrez with an eye on their midweek Champions League meeting with Porto.
And after United had gone close again when Ibrahimovic headed wide under pressure from Huth, the champions pulled a goal back when Gray's outstanding strike from outside the area flew past David De Gea and into the top corner.