Ferguson has also admitted that he wished he had signed the Dutchman when Peter Schmeichel departed in 1999, but United had to wait until 2005 to secure his services.
"Schmeichel was a warrior you know? He lambasted every defender in front of him, so aggressive. But Edwin was so calm, which was fantastic for our defenders, but he was very different in the dressing room," wrote Ferguson in his new book, My Autobiography.
"There was a magnificence and calmness about him and he was great for our players... I just wish I'd got him when Schmeichel decided to leave."
Van der Sar departed Old Trafford in 2011.