Amorim will take charge of the Red Devils on March 11 after his final game in charge of Sporting this weekend at Braga, but he could not have wished for a better send-off in front of the fans.
It was City's heaviest defeat since September 2020 and only the second time in his trophy-ladened spell at The Etihad that Pep Guardiola has been beaten in three consecutive games in a single season.
To cap a thoroughly depressing night for City, star striker Erling Haaland blasted a second-half penalty against the crossbar in a game that the visitors had led 1-0 through Phil Foden only to completely collapse.
Sporting's own Scandanavian superstar Viktor Gyokeres, who has been heavily tipped to join Amorim at Old Trafford, did much of the damage with a superb hat-trick.
The Swede drove home a 38th-minute leveller to and then kept his cool after the break to beat Ederson twice from the spot.
The first of those penalties came in a disastrous start to the second half for City as they went behind seconds after the restart as Maximiliano Araujo raced onto a through ball and fired into the corner.
The victory was labelled the greatest of Amorim's Sporting reign by media in the ground and gave United a taste of what to expect when the Portuguese takes over in just under a weeks' time.
Indeed, Amorim is set to go toe-to-toe with Guardiola again when United head to The Etihad in the Premier League on December 15.