The winger had come through the youth system at United before making his senior debut.
He recalled to the Broadtalks podcast: "I only signed a professional contract late. I remember earning £90 a week when I was 16 or 17 and the following year it doubled.
"The club wanted to give me a professional contract, but I didn't want to sign, because my father and I always thought that I shouldn't be a professional until I deserved it. I wasn't a regular in the first team or anything.
"It was strange because all my team-mates were signing their professional contracts and earning £1,000 or £2,000 a week and I was getting around £180.
"I will never forget my 18th birthday when I was on the bench in a Champions League game and the manager Sir Alex Ferguson knew I was the one who hadn't signed a professional contract.
"It was in added time with two minutes left and he said: 'Kieran, get on the field now. You don't have to warm up. Referee, do it now - get him in' because Fergie knew the bonuses United gave to win games.
"Back then I think we'd get £12,000 for a win and I was making £180 a week so I thought, 'S***, let me in' and when I went past him to get on the pitch, he said: 'Happy birthday my boy'. Bang, 12,000 pounds. It was wild."