An agreement with Sporting and Amorim was reached at the start of the month, but the deal was only made official on Monday, with the Portuguese giants fighting to keep him until the November international break.
Amorim signed off as Sporting head coach with a 4-2 win over Braga in Portugal's top flight on Sunday night, while Man United beat Leicester City 3-0 in the Premier League on Sunday in Ruud van Nistelrooy's final game in interim charge.
The 39-year-old has a huge job on his hands at Old Trafford, and while the club could be busy during the January transfer window, the majority of their business will take place next summer.
Amorim, who still needs to have his visa granted, will be tasked with making immediate changes, though, and with that in mind, Sports Mole looks at five priorities for the Portuguese, as he begins his work during the international break ahead of his first game in the dug-out against Ipswich Town in the Premier League on November 24.
Create a clear playing style
One of the criticisms levelled at former manager Erik ten Hag was that he was unable to implement a clear playing style, with Man United too often relying on individual moments of quality to win football matches.
Amorim simply has to create a clear brand of football at Old Trafford, and it will be fascinating to see whether he will implement his preferred 3-4-3 formation, which has proven to be so successful at Sporting.
Making such dramatic changes during the season will be difficult, but Man United need a clear style under Amorim, and there should be a noticeable difference if he does indeed decide to bring his 3-4-3 set-up with him.
Amorim will have time to settle into his new surroundings during the November international break, but he will then have a very busy fixture list, and there has to be a huge improvement to the football that has been seen throughout the season thus far.
The returning Luke Shaw and Leny Yoro could be crucial to his plans, as the former is the perfect player to operate as a left-sided wing-back, while the latter boasts the pace that will be required to play in one of the outside positions in a back three.
Address the issues in central midfield
Kobbie Mainoo is not too far away from returning, which will be music to Amorim's ears, and central midfield is an area that is in need of some special attention for the new head coach.
There has been clear improvement under Van Nistelrooy, but the team remain too open in the middle, with Casemiro and Christian Eriksen struggling in matches which become stretched due to their pack of pace.
Manuel Ugarte, though, has just started to show why Man United wanted him over the summer.
Indeed, against Leicester, the Uruguay international made eight tackles, which is the most of any Man United player in a Premier League match since records began; Amorim knows Ugarte well due to their time together at Sporting, and the 23-year-old could feature alongside Mainoo in what would be an exciting midfield duo.
Again, though, it will depend on the set-up in a game-to-game basis, and Casemiro has been one of Man United's better performers in recent weeks, with the Brazilian seemingly still having a role to play for the club moving forward.
Settle on a regular defence
If Amorim is to play a back three, then it has to be a settled one; Ten Hag too often made changes to the centre-back positions, even during matches, and that cannot happen if the team are to be successful.
Jonny Evans has again proven to be dependable this season; Harry Maguire has been out for some time and is still not back in training, which has limited the options somewhat, but the Englishman should be back before the end of the year.
When Yoro does return, it would be fair to assume that the Frenchman would be a part of the regular back three alongside Matthijs de Ligt and Lisandro Martinez, with Shaw playing down the left, leaving Noussair Mazraoui and Diogo Dalot to battle for the position down the right.
There will, of course, have to be changes during busy periods, but if players are to build relationships in a new system, then there has to be a sense of familiarity, especially in such a key area of the field.
Find a way to improve the team's finishing
Alejandro Garnacho has scored seven times for Man United this season, while Bruno Fernandes now has four, with the Portugal international regaining his form under Van Nistelrooy.
However, the lack of a prolific striker is still a real concern for the club; Rasmus Hojlund and Joshua Zirkzee have just three goals between them in 29 appearances, which is a concerning statistic.
Both senior strikers have just one goal in the Premier League this term, while no team has missed more big chances (26) than Man United in the Premier League this season, with the recent clash against West Ham United a clear example of that.
Amorim has left a club which boasted Viktor Gyokeres, with the Sweden international scoring an incredible 23 goals in 18 appearances in all competitions during the 2024-25 campaign.
The new head coach does not have the luxury of working with a free-scoring striker at Old Trafford, but he has to find a way to improve the team's output in the final third or it will cost them the chance to challenge for a European finish.
Sporting have been full of goals this season, hitting another four on Sunday to make it 39 in their 11 league matches, while they have registered nine in their four Champions League fixtures, including four against Manchester City last time out.
Help Rashford return to his best form
There have been some positive signs from Marcus Rashford in the early months of the 2024-25 campaign, with the 27-year-old managing four goals and three assists in 17 appearances, but his Premier League record is just one goal and one assist in 10 games.
Rashford has only completed the full 90 minutes in one Premier League match since the start of September, which is a concerning statistic, and it remains to be seen how the academy product fits into Amorim's new system.
Amorim, if he does play a 3-4-3, likes his attacking players to play inside, which might well suit Rashford, as he could be closer to the penalty box, but the manager also needs to fit Fernandes and Garnacho into the side.
Rashford is some way off the player that scored 30 goals and registered 11 assists in 56 appearances in all competitions during the 2022-23 campaign, and he continues to look frustrated.
A change of manager could be what Rashford needs to rediscover his best form, and it will be a big challenge for Amorim to help the Englishman come out the other side of what has been a tough period in his career.