That is the view of ex-Manchester City defender Danny Mills, whose former side were beaten 4-1 by Amorim and Sporting CP in the Champions League earlier this month.
Amorim has been hired by Man Utd to replace Erik ten Hag after the Dutchman was sacked midway through his third season in charge at Old Trafford.
Having taken charge of his final games in Lisbon, a spell which included that famous continental win over Man City, Amorim started his work at United during the international break.
His first game at the helm comes this Sunday away at Ipswich Town and while Mills acknowledges the task of rebuilding United's team is a long-term project, he feels there is still a certain level of results Amorim needs to hit in the 2024-25 season.
"Amorim needs to figure out which players fit his mould and buy into his project," former England international Mills said to Stats Perform.
"That will take six months or so. Then he will need to move some players out and bring new ones in.
"This season, though, they still need success. European football – Europa League – is probably mandatory. They must avoid the Conference League.
"Without being disrespectful, third-tier European football isn’t good enough for Manchester United, so it has to be the Europa League or Champions League.
"The Europa League is very achievable and not making it would be a huge disappointment.
"There are still good players at the club and they have still got to aim for the Champions League, but [getting into the top four] will take good coaching, a big turnaround, and maybe a bit of luck."
Amorim is the seventh different manager to take charge of the club since Alex Ferguson's retirement in 2013.
The 39-year-old won three Portuguese league titles with Benfica as a player while he left Sporting as head coach after guiding them to two league titles and two Taca da Liga trophies.
Sporting also started this season by winning all 11 league games under Amorim before his departure.
"The appointment of Ruben Amorim is a good one," added Mills. "You have to look at the people involved [in the appointment] as well.
"Dan Ashworth hasn’t made too many wrong decisions as a sporting or technical director. Throughout his career, at West Brom, England, Brighton, and Newcastle, his recruitment has been very good.
"Jason Wilcox, who sits under him, did incredibly well at Manchester City. He identified many young players who, if they didn’t become stars at City, went on to become stars elsewhere.
"Recruitment will be handled well in terms of managers, coaches, and players, but that takes time and it’s going to take at least two or three transfer windows.
"I like that Amorim came in and said, ‘Right, that’s it. A new start’. He has moved away from the Ferguson era that people keep harping on about.
"Ten-plus years ago, we knew how good Manchester United were, but football moves on. Liverpool went through a similar phase after their success in the 1980s."
Amorim takes over at United with the club 13th in the Premier League with 15 points from 11 matches and having collected just one win from four matches in the Europa League.
Mills believes there is plenty for United fans to like about the new boss' approach on and off the pitch.
"Football has evolved faster in the last 10 years than ever before and Amorim deals with the press well," he said.
"When Sporting beat Manchester City, he admitted they played well but got a bit lucky. People appreciate that honesty.
"Fans and pundits are more knowledgeable now than ever before, thanks to the amount of football they consume and the opinions they hear, especially on social media."
After facing Ipswich, Amorim's first two home games will see Man Utd take on Bodo/Glimt in the Europa League and Everton in the Premier League.
He will then take the Red Devils to the Emirates Stadium for a huge clash against Arsenal on 4 December.
United reached this season's Europa League by winning the FA Cup under Ten Hag and they have that potential path again this season, while they are also still in the EFL Cup, a competition in which they will play Tottenham in the quarter-finals.
Winning the Europa League itself, meanwhile, would mean qualification to the Champions League.
But while there are multiple routes open to Amorim in his bid to secure a continental spot, the Opta supercomputer does not like his chances of doing that through United's league placing.
After their poor start to the season, United have just a 13.2% chance of finishing in the top six, and a 33.1% shot at reaching the top eight.
As things stand, they are most likely to finish between ninth and 11th, meaning they are on track for their worst-ever Premier League finish unless Amorim can lead a rapid revival.