Van der Sar says City manager Roberto Mancini was brave to back Hart over Shay Given.
He told the Daily Mail: "It was a big decision by Mancini to put Hart in ahead of Shay Given because Given is a great goalkeeper.
"But someone had seen the potential and realised if they didn't give him the chance now, maybe the club would lose him.
"You have to sacrifice someone if you have a keeper you think is going to be even better.
"Hart makes good reaction saves and great blocks. That's a little bit the Peter Schmeichel way and that's one of his strong points."