The Dutchman spent six years at Old Trafford until his retirement in 2011, winning four Premier League titles and the Champions League.
"I spoke to Sir Alex over an unrelated matter about two weeks before," said Van der Sar.
"He is a great man and a great manager at a fantastic club.
"I wish him the best of luck and David Moyes also because it is almost an impossible job to step into.
"I am sure the manager and the players and the board are fully behind him. That is all you can ask for."