The Netherlands international opted for a switch to the North West from Arsenal and has since scored eight goals in his last 11 appearances for the club.
"I am glad Van Persie is there," said Van der Sar. "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."
Van Persie is expected to feature in United's Premier League clash against Chelsea this weekend.