Richards has been sidelined since undergoing surgery on a knee injury in October.
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."