"He should be one of my heroes because I was, and still am, a huge Man United fan," reveals McIlroy.
"But I remember I went and asked him for his autograph at the Portmarnock Links Hotel when he was with the Ireland squad and he just said 'no'. And it just sort of stuck with me ever since. So, that's why if a kid asks me for an autograph, I always try to do it."
Rory McIlrory's story clears up why he said a couple a years back that he'd love to "pay a lot to watch Sir Alex knock out" Roy Keane.
Rory says that the snub means that he will always do his best to give his own fans an autograph. A video of him signing a golf ball went for a young fan went viral last week.
"I was sort of surprised how that little moment last week became such a big thing because it's quite a regular occurrence on Tour," McIlroy says.
"I must use between six and nine balls a round, and every time I need a new one, I look for a young kid in the crowd in between holes to give it.
"It just shows what such a small thing can mean to a kid. I had it go the other way for me, where I asked someone for an autograph as a kid and they didn't give it to me. And I've never liked them since."