Early indications following the defender's knee injury suggested that he could return to training within eight weeks.
However, having undergone surgery, the 24-year-old has admitted that the complaint is worse than was first feared.
"The positive thing is that the injury has been repaired so the knee will be as good as new," he told the club's official website. "I have a brace on it and I am using crutches. I can't do anything at all for the next four to six weeks and I am probably looking at 12-16 weeks before I am back in action.
"I knew straight away in the game that it was quite a bad injury. The knee locked and I couldn't straighten it.
"I suppose it could have been even worse and not repairable which would have meant removing the meniscus; so I am being positive. Injuries are all part and parcel of the job but this coming on top of the other one I do feel a little hard done to.
"I will be working as hard as I can to get back as quickly as I can and I thank all the fans who have sent their best wishes."
Richards joined City in 2001 as a trainee.