Though the Dutchman is now 38, the Manchester United boss has confirmed he will never be hounded out of Old Trafford because his form is phenomenal.
But Fergie's insistence on making Van der Sar first choice has stifled Ben Foster's World Cup chances.
The England hopeful faces spending the rest of the season on the bench as Ferguson refuses to allow him out on loan.
And a lack of regular action means Foster will fall foul of Fabio Capello's rule of only picking players in form.
Ferguson said: "Ben was always going to lose his place when Edwin Van der Sar came back.
"You don't dismiss the quality and experience of Van der Sar. You couldn't even say he is declining - he is at his peak.
"He is phenomenal. He really is top dog.
"He is in training every day. He doesn't miss a session. Plus he has that calmness about him. He never gets excited about what is happening round about him.
"There are not many in England you can compare him with.''

Foster could be forgiven for working up a nervous sweat after hearing Fergie's recent praise for CSKA Moscow's giant keeper Igor Akinfeev.
But Foster has been assured he is still in line to take on Van der Sar's crown when he eventually quits.
Ferguson said: "The future belongs to Ben of course. Tomasz Kuszczak is a good keeper too so we are quite strong.
"But Ben will not be allowed out on loan in January. No way.''
Foster could even be promoted in the summer if Van der Sar decides to hang up his gloves.
Fergie insists he is no wiser than the rest of us on the veteran's future plans.
He added: "He makes his own decisions about that. There is no point me trying to persuade a man of 38 that he can carry on or should finish.
"He is a great professional and a sensible man. He has his own thoughts on what he wants to do, how long he wants to go on for and what his future is.
"He just comes to me once a year and says he wants another contract and we'll stick with that.''
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