The pair played together for the Dutch national team.
And Van der Sar told the London Evening Standard: "I am glad Van Persie is there.
"It is always nice when someone like that decides to go to the club. I spoke to him about it and I know it is the right place for him. He can really add something to the team.
"It was difficult for him at first because he didn't know if he was running into the right areas. But he has settled in very well.
"He is a great guy to have around. He works so hard on and off the pitch.
"I know there has been a focus on youth at United but he will improve the team."