Hart has been left on the bench by new boss Guardiola for City's first two games of the season and the England goalkeeper is increasingly being linked with a move away from the club he has been with since 2006 and made close to 350 appearances for.
Rangers midfielder Barton told talkSPORT on Wednesday: "It is not the club I left.
"The club I left and watch now are two totally different organisations. The club I played for wouldn't have treated a player that had been a servant for as long as Joe has in the manner that it has. I think it is disgusting.
"He is a full international, somebody who, probably before City took the money, could have gone on to bigger and better things himself but stayed and wanted to be at City.
"I don't see what he has done wrong to be treated the way he has. His attitude isn't terrible. I don't like this.
"It is common human decency, regardless of how good a coach you are. Why treat him like that?"
Guardiola started Willy Caballero ahead of Hart both in Saturday's 2-1 win over Sunderland and Tuesday's 5-0 triumph at Steaua Bucharest.
Barton added: "I've watched Caballero and are you telling me Caballero is better with his feet than Joe Hart? I haven't seen it.
"Why not let him fight for his place and then if he is not good enough say: 'I have given him an opportunity'. That's what every manager should do, give everyone an opportunity."