The Scot, who has been at Old Trafford for over 26 years, caught many by surprise when he announced on Wednesday that he would be stepping down at the end of the season.
And he has admitted that he struggled to keep the news under wraps, having not even told members of his immediate family until the official announcement was made.
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"She's isolated a lot now and I think I owe her a lot of my own time. For 47 years she's been the leader of the family, looked after our three sons, sacrificed for me, and now she's got all her grandchildren: they all dote on her, and I think you know, she's lost her best friend, her sister Bridget, and I think I owe her my own time.
"That was important. Also, to go out a winner, that is really important at this club. That's the most important thing I ever wanted to do here, to be a winner.
"It was very difficult. Sometimes I nearly blurted it out to the family. We told our sons around about March. My brother didn't even know until about Tuesday night, because I wanted to tell the players first really, the players and my staff.
"But unfortunately those rumours came out - where they come from, you never know. This club's like a sieve. Stuff leaks out that shouldn't leak out, so we started to speed it up a bit."
Sir Alex also confirmed that Wayne Rooney has asked to leave Manchester United.