Richards has been sidelined since undergoing surgery on a knee injury in October.
The England international had been making only his fourth appearance of the season, after missing the start of the campaign with an ankle problem, when he tore a meniscus in a game against Swansea.
The 24-year-old was initially ruled out until the end of February but, despite returning to training, is not yet ready to play.
Mancini said: "This is strange. I don't know how it is possible, this. But we can do nothing. I think maybe in three or four weeks he will be ready.
"I don't know (the problem). His knee was not 100 per cent. He continued to work. Now it is two months that he has been working."
Mancini added that Richards has trained on the pitch but has not yet rejoined the first-team squad.
When asked if he thought Richards would be playing again by now, Mancini said: "Yes, maybe two or three weeks (ago).
"When he starts to work with the team he needs another two weeks. I hope he can start with the team next week."
It is not the first time Mancini has expressed frustration with Richards' fitness record after a number of niggling problems in the past.
After Richards suffered his latest injury, Mancini also suggested he felt the player should be fit again within a month.
Mancini was also forced to defend his tactics following the Champions League loss at Ajax last October after Richards said the players were unfamiliar with the system used.