The disgraced Chelsea captain retired from international football days before being found guilty by the FA of using racist language towards Anton Ferdinand in a league match.
"John is a great centre-half, a great servant to his country, an inspiration and a very passionate and proud Englishman," said Man City goalie Hart.
"It's a shame we've lost him as he's still got so much ability. It's a big blow, but it's John's decision. I don't know what's going on in John's life on or off the field.
"I don't know his body or his frame of mind. I only know that, when I'm here with England, training and eating chicken and pasta next to him, he's a really good guy, a good person and, football-wise, an inspiration."