Rashford has joined Barcelona on loan after being bombed out by Amorim following a tumultuous spell in his career.
Amorim previously claimed he would rather play his 63-year-old goalkeeping coach than Rashford, before Aston Villa handed him a loan deal in January.
United endured their worst-ever Premier League campaign after Amorim replaced Erik ten Hag last November and Rashford told Gary Lineker’s Rest is Football podcast: “People say that we’ve been in a transition for years but to be in a transition you have to start it. The actual transition hasn’t started yet.
“When Liverpool went through this, they got [Jurgen] Klopp and stuck with him. They didn’t win in the beginning. People only remember his final few years when he was competing with City and winning the biggest trophies.
Barcelona's on-loan striker Marcus Rashford celebrates with Raphinha
Barcelona's on-loan striker Marcus Rashford celebrates with Raphinha
Bryan Mbeumo completes move to Manchester United from Brentford
“To start a transition you have to make a plan and stick to it.
“This is where I speak about being realistic about what your situation is. We’ve had that many different managers, ideas and strategies in order to win that you end up in no man’s land.”
On the pain of United’s decline and whether it hurts, he added: “Yeah, 100 per cent. But not just as a player but as a fan.”
Barcelona will cover the entirety of Rashford’s salary for this season which will save United around £12.75m in wages. The Spanish champions also have an option to buy Rashford next summer.
He played just six games under Amorim last season before being frozen out. Amorim questioned Rashford’s attitude and application in training, while club legend Teddy Sheringham said the 27-year-old “did not deserve” a move to the Camp Nou.
Rashford has also claimed the club have been held back by being too “reactionary” and not sticking to the principles set in place by Alex Ferguson.
Manchester United scarves, with new signing Matheus Cunha (left) and Marcus Rashford, who has recently be loaned to Barcelona, on sale outside Old Trafford
Manchester United scarves, with new signing Matheus Cunha (left) and Marcus Rashford, who has recently be loaned to Barcelona, on sale outside Old Trafford
“When Ferguson was in charge, not only were there principles for the first team but they were for the whole academy,” he added.
“So you could pick players from 15 years and they’d all understand the principles of playing the Manchester United way. Any team that has been successful over a period of time has principles that mean that any coach or player that comes in has to align to or add to the principles.
“At times, United were hungry to win but it was reactionary. If your direction is always changing you can’t expect to win the league.”
United have splashed out over £200m on signings this summer with the arrivals of Matheus Cunha, Bryan Mbeumo and Benjamin Sesko. They open their campaign against Arsenal this Sunday.