The midfielder is one of the great modern myths of United's academy.
McGuinness told the UTD Podcast: "He was an amazing player.
"And the best way to coach him was not to coach him.
"The best way was to create a game where his stuff would come out. If you start telling him what to do, he wasn't go to be having it, you know? So he was an amazing player and I've had some really top players so it means that, as a coach, you learn from your players as well.
"So, in the sense that (fellow coach) Jim Ryan had a phrase. He said it's the 'arrogance of timing' and so some coaches in the position thought he was being arrogant. But it was just the way he played.
"It was like Messi," he recalled. "He'd wait for you, wait for you to go closer and closer. Close to the fire, without getting burnt and get close, then he'd just play it. And you'd be like 'oh, you'd better pass it now' and he wouldn't.
"You couldn’t tell him when to play it. No, he was too good for that, in that sense. So, yeah, he was amazing. I remember we won the league and then we won the Youth Cup and we played against Leeds in like the last (league) game, I think, and they didn’t see the ball for 10 minutes.
"And he was unbelievable and then, all of a sudden, the centre-half just kicked him from nowhere and the coach is on the side, Neil Redfearn, and said: ‘About time too!’
"He was like, somebody do something because he was too good! And he could do those things with the first team and a lot of people say: 'Oh yeah but he’s thrown it all away, he’s wasting it and so on'.
"But, actually, if you know the full story from being younger, his background and different things, he may actually be one of the best performing. You know, he might actually have done well despite all the sort of disadvantages he had."
Now 31, Morrison last played for DC United in 2023.