"It's not as bad as was first expected," Richards told City's official website.
"There's still a bit of inflammation and swelling of the knee, so it's for the best to just rest it over this period."
Richards has been overlooked by Fabio Capello since the Italian took over from Steve McClaren at the end of 2007, but harbours hopes of forcing his way back into the senior England set-up.
He said: "It's a bit frustrating and I'd be lying if I didn't say it was frustrating. I'll just bide my time and when the time's right hopefully I'll be able to get back in."




