The Portuguese coach is on the brink of taking the reins at Old Trafford as the new permanent manager following the sacking of Erik ten Hag. Merson has lauded the two-time title-winning coach, but warns that the high-pressure environment at United will present a whole new test for Amorim.
"He's done amazing at Sporting," said Merson. "This is going to be different. No disrespect. They play against Porto and Benfica and that's it. He's coming in to manage one of the biggest clubs in the world. We've seen it with Ten Hag: brilliant at Ajax, come to Man Utd, win one, draw one, lose one and you're under pressure. There's a lot of work to be done. This is five steps up.
"Will Amorim be different to the managers who have gone before? I don't know if he will. He's managed in Portugal, done everything right, his team did very well against Arsenal last year. But this is different. It's hard to explain.
"We've seen with all these managers, you're under severe pressure every game. As a football team they're nowhere near those heights. One of the biggest football clubs in the world with a just-above-average football team. I can't emphasise enough how far they are behind the big boys at the moment."
TrendingMerson says the scale of the task facing Amorim - with United currently seven points off the top four - means qualifying for the Champions League is almost out of reach. In fact, Merson believes it would be a stunning season of coaching from the incoming boss if he were to deliver a top-four finish this term.
"I'd be surprised if they went top four," said Merson. "You could put Pep Guardiola in charge of Man Utd and he couldn't turn it around that much. It's going to take time.
"They have to build and build whereby in three to four years time United are back where they should be, fighting for Premier League titles. They have to be patient, bring players in steadily.
"If they get top four he'd have pulled up trees and should get knighted by our king!"
Merson says Amorim's priority when he arrives is to coach the players into a clear style of play, something he believes Ten Hag never achieved during his tenure.
"Getting the players to understand the way they want to play," said Merson when asked about Amorim's first main goal at United.
"I watch Man Utd play and there hasn't been identity there for a long time now. He has to come in, get his ideas across and start playing as a team. They don't press together. He has to get everybody understanding the way he wants to play.
"I don't think anyone could sit there and tell me what Man Utd were trying to do. Chelsea against Newcastle, you know what they're trying to do. That manager has been in there two or three months.
"They've got to learn how to walk first. There are going to be some great times in the next two or three months where they play scintillating football and win games but it's going to be up and down. I don't know how long it is going to take but it won't happen overnight on a consistent basis."
Another key for Amorim will be making the right moves in the transfer market, says Merson.
"Ten Hag got the transfer market horribly wrong. I'm trying to think who he brought in who was a major success. They bought Antony for £80m... That's a sacking in itself!
"They will need pace at the back [to play Amorim's preferred 3-4-3]. They will push their wing backs on to help the two in midfield. That's where they'll probably have to recruit: pace at the back.
"And they need a goalscorer. The two lads up front haven't done it. Rasmus Hojlund is a young lad and Joshua Zirkzee hasn't been given much time yet but they need a goalscorer."