Spain captain Casillas' future has been the subject of speculation with the shot-stopper finding himself out of favour at the Bernabeu under new coach Carlo Ancelotti just like he did at the back end of Jose Mourinho's reign.
Hart has come under pressure after two mistakes in the midweek UEFA Champions League defeat by Bayern Munich and City have been credited with an interest in Casillas.
However, Argentine defender Zabaleta does not believe City need Casillas at the Etihad.
"At Manchester City we do not need Casillas," Zabaleta told Marca. "He's been linked with other clubs.
"I hope he stays in Madrid."
Meanwhile, City team-mates Micah Richards and Samir Nasri have backed Hart to bounce back from his below-par showing against Bayern.
"He will bounce back because he is a strong character," said Richards. "He knows he could have done better. We all make mistakes."
Nasri added: "I think that when you are a top player like Joe, you don't have problems with confidence.
"Everyone is behind him at the club."