Leicester imploded defensively as United racked up four goals between the 22nd and 42nd minutes.
It was a stylish display from a United side who had entered the game under pressure after losing their last two league games, against Manchester City and Watford, with Chris Smalling opening the scoring after replacing Rooney as captain.
Juan Mata, Marcus Rashford and Paul Pogba, with his first United goal following his world record £89 million arrival from Juventus, were also on target after England captain Rooney was dropped to the substitutes’ bench.
But it was a chastening afternoon for Leicester despite an improved second-half display and a brilliant consolation goal from halftime substitute Demarai Gray.
It was the second successive Premier League away game in which Leicester had shipped four goals after doing so at Liverpool two weeks ago.
Leicester held their own early on with Robert Huth lashing a shot just over but it was United who edged in front in the 22nd minute after some neat play wide on the left had forced a corner.
It was delivered by Daley Blind and Smalling escaped the United defence to power home a header from six yards.
Rashford should have doubled United’s lead moments later when Leicester squandered possession and Zlatan Ibrahimovic galloped forwards and squared for the teenager.
But Rashford’s first touch was heavy and his shot flew over.
Ibrahimovic then went close to a goal after collecting a fabulous pass by Pogba, but the Swede’s effort on the turn just cleared the bar.
Next, it was Pogba’s turn to let fly with an outrageous effort from 30 yards that bent towards the top corner but was forced away by Leicester goalkeeper Ron-Robert Zieler.
United finally added a second goal as they turned on the style eight minutes before halftime. – AFP