Greening was snapped up from York City.
He told GrandNational: "It was such a strange thing because I was at a little club called York City, who were in League One at the time. I'd just signed my first professional contract as a first-year pro and played a first team game against Fulham and I didn't play that well at all!
"The manager pulled me in for a chat after the game, I thought he was going to give me an ear bashing because I played quite bad. He said, 'Sit yourself down, I need to have a chat with you'. I'm thinking 'Here we go…' But then he said he had spoken to Alex Ferguson on the phone who wanted to know if I could train with Manchester United for 4 days on a trial. I was like 'Is this a wind-up?!', and then realised it actually wasn't.
"I went to Manchester and trained with the first team, Becks, Giggsy, Keane, as a fan, the whole thing was unreal. Sir Alex Ferguson thanked me for coming and said I was brilliant and that it'd been one of the best trialists they'd had from a kid coming in from another club.
"He (Sir Alex Ferguson) sent me back to York, and on my way back I was thinking that must have been Sir Alex's way of saying 'I was okay, but not good enough for Manchester United'. Then two weeks later they put a bid and it was all agreed and I ended up going to Old Trafford.
"It was surreal, I was still 18 and to go from such a small club like York City to one of the biggest clubs in the world, playing with the likes of Ryan Giggs, David Beckham, Roy Keane, the Neville brothers and Andy Cole, it was crazy! The first couple of months was weird because you see them on telly and then you're training with them every day.
"I followed Steve McLaren who was my assistant manager at Man United and he wanted to take me to Middlesbrough, so I ended up going there in 2001 with him."