Johnson was regarded as a huge prospect at City, but had his contract cancelled by the Premier League champions after the two sides agreed on a severance package from his £40,000-a-week deal.
The England Under-21 star was expected to make the transition to the senior side but his career at both club and country level fell apart as the midfielder fell out of shape.
Still only 24, Johnson told the Manchester Evening News that his decline stems from mental health problems which led to treatment at the Priory Clinic.
"I have been attending the Priory Clinic for a number of years now with regard to my mental health and would be grateful if I could now be left alone to live the rest of my life," he said.
"I am more disappointed than anyone, but that's the way it goes."