The 16-year-old centre-half has signed a youth deal with the League One Millers and is described by the first-team boss as “a wonderful, wonderful talent”.
But Harrison, who was playing for Doncaster City in the Central Midlands Alliance Premier Division North, might never have come to the club’s attention had it not been for a motorway snarl-up last month.
“Basically, it’s down to an old guy who has found some really good players for me in the past,” Evans said.
“He doesn’t take a penny for it, he just takes pride and joy going around matches all over Yorkshire.
“He was at a Doncaster match by chance because he couldn’t get to the game he was meant to be going to because of traffic problems.
“He phoned me up and said: ‘I’ve seen this kid. They tell me he’s 15 and I can’t believe it.’”
Evans says he allowed the youngster, who can also play in midfield, to join in first-team training so that he could assess his potential before involving Rotherham’s youth set-up.
“Harrison was very, very good, I then handed it to the academy,” the boss said. “You have to let the judgement and process go from there with (academy manager) Richard Hairyes and his staff.
“Within two sessions they were ringing up saying: ‘We’ve got to get this kid signed.’”
The plan is for Duncan to continue to experience the first-team environment from time to time.
However, Evans stressed that fans won’t be seeing him in a matchday squad for a third-tier fixture any time soon.
“He’s a wonderful, wonderful talent, but that’s all he is right now,” the manager said. “He’s not ready to be playing against, say, Jay Stansfield, he’s not ready on a regular basis to be playing against someone like Jonson Clarke-Harris.
“You have to think that six weeks ago this kid was 15. He’s not 17 until next August. When we have him up here to train with the boys, it’s really good for him because it will make him better.”
Assistant boss Paul Raynor, speaking after the 2-2 draw with Burton Albion on September 14, didn’t rule out an appearance for Duncan in a Bristol Street Motors Trophy group-stage game.
Evans said: “He looks a tremendous prospect, we’ll see where his journey goes.”
