Despite shining as a midfielder last season, Mainoo has been deployed in a more advanced role under Amorim, even leading the attack against Crystal Palace.
His recent stint as a No. 10 in the FA Cup win over Leicester saw him struggle to make an impact.
Ex-Chelsea ace Cascarino stated on talkSPORT: "When he (Mainoo) first came into the Man United team, I thought he really did it at a breeze.
"It was quite comfortable for him, he felt confident on the ball, he knew what he was wanting to try and achieve.
"Gets him in the England squad, does really well in the Euros, plays well, doesn't look out of place, really high standard. I'm watching Kobbie Mainoo last night, and it's not on him, I'm putting this on the manager and the coaching team around him.
"I think he looks a confused footballer now. Because Kobbie is going backward too often. I think he feels he's afraid to lose the ball, and that's what can happen when you get into a really technical setup.
"Which is what Amorim is trying to achieve. I'm watching Kobbie Mainoo, and I think he doesn't really know what he's meant to do now. I feel sorry for him because he's 19, and he's learning on the job.
"When he first broke in, everything about his game was spot on. He's gone from a central midfielder, to an attacking midfielder to a centre forward. He's the future of Manchester United. He's in a position that isn't natural for him.
"Today, we choose to play with Kobbie (Mainoo) to use the characteristics of Kobbie. It's not nothing against Rasmus or Josh. And, sometimes, the team scores goals by creating situations, to use the right characteristics," Amorim said after last week's loss.
"It doesn't mean if you play with two strikers you are going to score more goals. So I think it's a problem for the team and we have spoken about that all the time. We need to improve our performance in every aspect of the game, especially scoring goals."