The Sweden midfielder is understood to be close to agreeing a move to Liverpool.
"We cannot afford to lose Rasmus on a free transfer," Stewart told Voetbal International. "It goes without saying that we want to keep him happy and to stay here longer.
"We are even talking to eachother, but with the birth of his daughter these discussions were postponed.
"Despite all the interest for Rasmus, I really hope he stays at AZ."