Mainoo was seen as a potential superstar after his first couple of seasons at United, but his role diminished severely under Amorim. The midfielder didn't make a single Premier League start under the Portuguese boss this season, and played over 30 minutes in just two games.
Under Carrick, Mainoo is back to being a prominent member of the side, playing the full 90 minutes in every game under the interim boss, and assisting twice within them.
Carrick has told BBC Sport of the trust he's putting in the 20-year-old.
He said: "I've known Kobbie a long time. I started working with him when I think he was 13 or 14, when I was starting to do my coaching badges - a good few years ago. Just little bits. And then, obviously, when I was here the first time, he was in and around a little bit.
"I said earlier about coaches being able to cope with being here and at the level to deal with it. What Kobbie's done at such a young age is quite incredible really.
"We forget how young he still is. I was just a big fan of watching him play and knowing what he was capable of. So it wasn't really a big decision to play him. And, to be fair, it's not easy when you haven't played to find your rhythm and find your form.
"There are things he can get better at, things he can improve on, but we haven't really got started on any of that because we're just letting him go and find his flow and find his rhythm of playing football again.
"I've been really conscious of not giving him an awful lot - a couple of little pointers, a bit of positional things and some little bits here and there - but trust in what he is. He's a fantastic footballer and he's got a huge talent."
It's not lost on many in the game how poor a call if was from Amorim to keep Mainoo in the cold prior to his dismissal earlier this season.
Gary Neville recently stated that the midfielder being left out of the side was the "worst decision Ruben Amorim ever made."
The United legend felt the treatment "killed" Mainoo, so it'll be a relief to many that he's back in the mix now.
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