The England international, making his fourth appearance of the season after recovering from an ankle problem, suffered a serious knee injury in a 2-0 victory over Swansea City in October.
The 24-year-old was forced to undergo an operation on what was deemed to be a torn meniscus, and despite returning to training in February, Richards is struggling to gain full fitness.
“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,” said Mancini.
“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 hopes to have Richards available for the title race’s climax, but the Italian conceded the defender would still take at least another three weeks to be match fit.
“When he starts to work with the team he needs another two weeks. I hope he can start with the team next week,” he added.