Van der Sar says they spoke about two of his former clubs in Juventus and United, both of whom were keen on the player - but United dominated the conversation.
"He called me up and said other clubs were in for him, but because I played for two of them, United and Juventus, he wanted to hear my thoughts about them," he is quoted by the Daily Telegraph.
"So we spoke for five minutes about Juventus and 55 minutes about United.
"He wanted to know how good United were, what he would find there, how the players are, what Ferguson is like, how they train, how the system works.
"He has shown his quality already. I read a statement from Ryan Giggs, saying that United have signed good players and good talent before, but that this one [Van Persie] has everything.
"He has the whole package already, but that doesn't always mean it will work of course. Look at Fernando Torres at Chelsea. His first year was difficult, but I think the signs with Robin are good, though."