Carrick brought Mainoo back into the starting XI after the midfielder had been frozen out by predecessor Ruben Amorim.
"I've known Kobbie a long time," he told BBC Sport. "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.
"So I think knowing him and having experience with him and seeing him perform at such a level on such big occasions... 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."