Darren Ferguson insists his father's retirement had been planned for some time and has nothing to do with any health problems.
Sir Alex Ferguson will take charge of his final game at Old Trafford on Sunday before addressing the crowd following the Premier League encounter with Swansea.
It promises to be an emotional day, with the Scot's 13th championship trophy also to be collected, and feelings will be heightened because of the sudden nature of his exit.
So far, the man himself has handled the situation in the same clear-sighted way his manages his team, his son said.
"He said: 'I've got to tell you, I'm doing this, this is why I'm doing it.' And that's it," Darren Ferguson told twentyfour7football.com. "His last game on Sunday at Old Trafford could be a bit emotional because it's my dad it is what it is, no one will ever replace him. But he's ready to go."
Most assume the decision has something to do with the 71-year-old's health given he has always said that would be the determining factor in any decision to quit and it was confirmed last week he would have a hip operation in the summer.
However, Darren Ferguson, who manages Championship side Peterborough United, is adamant that is not the case.
"There's no problem with his health, contrary to what some people are saying," he said. "He's got a small hip operation but there's no difference to most 71-year-olds. It's not a health decision. Basically he feels he's gone out at the right time, he's left the club in a fantastic position and they've hired a very good manager to replace him."
Information from Press Association was used in this report.