Ferguson believes that Van der Sar would not want his form to drop and also confirmed he wanted to spend more time with his wife Annemarie, who suffered a brain-related illness last year.
"I have come to accept this will be Edwin's last season," Ferguson told The People. "It's sad but the difficulty for a goalkeeper when he gets to 40 is something which happened with Peter Shilton.
"All of a sudden, because of age, Shilton's form just collapsed and I don't think Edwin would want that. He is such a proud man and it's going to be a big loss.
"We had a chat last season about how he saw his future and he explained to me his plans, and nothing has changed from then.
"It's difficult because he has his wife's health to consider after her illness.
"She is fine now and seems to be really good but she still has treatments to get in Holland, with physiotherapy and things like that. So for him to be here with us would be difficult."