"It's not easy," van der Sar explained in an interview with FATV.
"He moved for a certain reason, and that was to be involved in title races and trying to win trophies.
"The destination he chose - to go to Man United - they're always in the hunt for more.
"Their players have won loads of things already but they're still hungry for another title.
"Let's hope they can add something to the silverware this season."
United take on West Ham at Old Trafford in an FA Cup replay on Wednesday night, and van der Sar believes his old club dodged a bullet after van Persie's late strike secured a replay.
"I think they're very fortunate to still be in the tie," he said.
"Hopefully they can finish it off at home, but over the years United hasn't been that successful in the FA Cup.
"I think first they're looking to get it out of the way and for the first time in ten years win the FA Cup again."