Wayne Rooney equalled Sir Bobby Charlton's goalscoring record as the rotated Reds put Jaap Stam's Royals to the sword and sealed an FA Cup fourth-round spot.
Stam - serenaded by both sets of fans throughout - enjoyed a stellar, if brief, Reds career in his playing days but was unable to prevent his second tier side from causing a huge shock at the hands of the holders.
Rooney - one of nine alterations in a much-changed United side - moved alongside Sir Bobby with his 249th goal inside ten minutes.
Anthony Martial doubled the lead and Marcus Rashford netted his first goals since September as the Reds - despite resting Paul Pogba, Zlatan Ibrahimovic and Henrikh Mkhitaryan - were simply too strong for their Championship visitors.
United started on the front foot and could have been ahead inside a minute.
Marcus Rashford lashed a shot wide before Al Habsi saved well from Rooney after Daley Blind's delightful through ball had picked him out.
That proved to be only a pre-cursor, though, as the skipper marked his return with a historic moment in the seventh minute.
England's all time record goalscorer diverted home Juan Mata's cross with his knee eight yards out to bring up his landmark United goal.
Reading, third in the Championship, struggled to gain a foothold in the match and found themselves 2-0 behind on 15 minutes.
Martial played a lovely one-two with Rooney on the edge of the box and the Frenchman stroked a shot through the legs of Liam Moore and beyond Al Habsi for his sixth of the season.
Marcos Rojo went off through injury but United continued to swarm forward and could have increased their lead tenfold by the break.
Martial's 20 yarder almost slipped under Al Habsi, Rashford fired into the side netting from a tight angle and both Rooney and Mata went close from distance.
The Royals keeper continued to keep the Reds at bay as he pushed away Michael Carrick's curler shortly after the break and then also did well to deny Marouane Fellaini and Rooney in quick succession.
Martial's dangerous cross narrowly evaded the onrushing Rashford as Reading - Cup semi-finalists two seasons ago - continued to be on the back foot.
Stand-in stopper Sergio Romero saved at the feet of Garath McCleary but United were dominant and moved 3-0 ahead on 75 minutes.
Rashford got the goal his performance deserved when he raced on to Carrick's brilliant through ball and fired in his one-on-one finish with aplomb.
Bastian Schweinsteiger received the biggest roar of the day with his 78th-minute introduction before Rashford netted his second and United's fourth in comedic fashion.
Al Habsi mis-kicked Moore's backpass to present the striker with the easiest goal he will ever score to add gloss to the scoreline against a plucky but outclassed Reading outfit.
Overall team performance: 9/10
United Faithful Man of the Match: Marcus Rashford