The pair played together in Spain.
Heitinga, who admits to whacking him in training without a whimper from the Argentina striker, said: "He's unbelievable. I told my colleagues when City signed him that he's one of the best players I have ever played with.
"Sergio could be the difference to City winning the title or not. He's that important. They have got a few really good players but he is one special player."