The England defender has suffered a series of injuries over the past 12 months.
And his latest calf strain has been linked to a long-standing back problem that has placed question-marks over his future at United and with England.
Ferguson said: "Rio isn't far away now. He is doing much better and thinks he is at the level he was when he was playing last time. But we have got to handle it correctly.
"He is seeing the right medical people and I would expect him to be back before Christmas.
"I don't think we would consider an operation. You don't operate on a back unless it is really serious and it's not that bad."