The Scotsman won 13 Premier League titles at United, along with two Champions League trophies and countless domestic cups.
He spoke with Ally McCoist about the fact that he had to give up his professional life for family reasons.
He stated, per The Sun: "(His late wife) Cathy’s sister had died in the previous October and she was lost, she was definitely lost.
"So at Christmas I said to her, 'Look, I’m going to retire at the end of the season.'
"I knew by her reaction that she was delighted because she was on her own.
"She had given her life to me, everything to me, looking after me because of my job… bringing the kids up.
"When I said I was going to retire, she was delighted and I knew that. I could tell by her body language.
"I always said when I got to January – what I did every January 1, I wrote down every year where our opponents’ games could be won or lost and I put us down to win by ten points.
"The team was playing really well and I thought we’d do that, but I think it was 11 points.
"They did really well, the team did really well and Robin van Persie was fantastic – his goals, his thrust in the game, his power and confidence.
"The team had perseverance, we never gave in, that’s the quality that great team always had. We had that for years.
"At the end of the day when you look at my career at United, it’s encapsulated by the leagues, but it’s also about myself and the team – we were second in the league six times and won it in the next year.
"Isn’t that amazing? Six times second, twice on goal difference, and we won it in the next year.
"That tells you what that club is about, what these players are about – not just one set of players, that was going back to the 1994-95 team – tough as nails – to the really talented team in 2008 with Carlos Tevez, Cristiano Ronaldo and Wayne Rooney up front. They were unbelievable."