The 24-year-old had an operation to repair a torn meniscus late on Monday night after sustaining the problem in Saturday's 1-0 Premier League win over Swansea.
It could now be as late as February before Richards is seen in a City shirt again.
“The positive thing is that the injury has been repaired so the knee will be as good as new,” he said. “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.”
The defender sustained the injury when attempting to clear from a cross and required oxygen on the pitch before being taken off.
The lay-off is yet another setback for Richards who missed the start of the season with an ankle injury sustained playing for Great Britain during the London 2012 Olympics.
He made his return earlier this month in the 3-0 win at home to Sunderland but after just three further appearances he finds himself sidelined once again.