The 40 year-old Dutch goalkeeper announced his decision to retire at the end of the 2010/11 season on 27 January but this week, with the veteran's final games looming, he started having second thoughts and went to see his manager.
"Edwin decided he wants to go out at the top, though he did have a wobble last week," said Sir Alex, in his exclusive column for United Review - the Reds' official match day programme.
"Edwin came to see me and told me he was thinking about changing his mind. I had to tell him to be quick about it because we are in the middle of concluding a deal for a new keeper.
"He came back a couple of days later to say he would be sticking with his original decision to go, which I think at his age,41 next birthday, is the right thing for him to do."
Van der Sar consequently remains on track to make his final competitive appearance in the Champions League final against Barcelona on 28 May. The corresponding fixture in 1999 was Peter Schmeichel's last game for Manchester United, although the Great Dane went on to play for Sporting Lisbon, Aston Villa and Manchester City before he retired.