The 71-year-old officially called time on his 27-year reign at Old Trafford at the end of last season, but had made the decision to step down in February.
The Scotsman has revealed that he worried for the club's status when he discovered that chief executive David Gill had also decided to leave his role at the same time.
"He came over to talk to me about him retiring himself," Ferguson told MUTV. "I says, 'Christ, I've made up my mind to retire myself at the end of the season.' That gave me a terrible quandary because you start thinking about leaving the club in the lurch.
"I said, 'I'll need to speak to Mr Glazer, to Joel,' and he organised for Joel to give me a call and I explained to him why I wanted to retire and why it was the right reasons and he accepted that.
"That was at the end of February. So that period was difficult. I was trying to keep it quiet and couldn't tell some members of my family."
Ferguson's new autobiography is due to be released this week.