The Bulgarian winger joined Wanderers on a free transfer in the summer after his contract at Eastlands was allowed to run down.
And he says his final memory of the club is one of unhappiness.
Petrov said: "I don't like saying bad words about City because they gave me my chance to make my name in the Premier League.
"But they have so much money now that they think they can go out and buy everyone.
"We saw in City's last game [the Carling Cup defeat by West Brom] that money isn't everything.
"If you don't have a team, if you don't play together then you won't be successful.
"Of course I was unhappy at the end, but it wasn't just me who was unhappy - it was everyone.
"Everyone became unhappy with the situation at City."