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.
Meanwhile, reports have suggested Barcelona keeper Claudio Bravo is set to complete a switch to the Etihad Stadium.
Rangers midfielder Barton, the City academy graduate who spent five seasons in the first team before departing in 2007, 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, the former Barca coach who took over at City this summer after leaving Bayern Munich, 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."
Source : PA
Source: PA