Rio to return against Hull
The centre-back has been sidelined with a back injury since playing against Liverpool on October 25.
"Rio will play tomorrow which is good news," Ferguson, who has had to deal with a lengthy injury list in recent weeks, told a news conference on Friday.
"He has been training very well the last few weeks and this is the time to bring him back now.
"The defence has been the weakest part of our game this season, losing (Edwin) van der Sar, (Nemanja) Vidic, Ferdinand, Gary Neville for a period; John O'Shea, Wes Brown and Jonny Evans have all been injured at various times.
"We hope to see Vidic, in the next couple of weeks, join that list and that can only be good for us."
O'Shea is likely to miss the rest of the season due to a leg injury.
With leaders Arsenal and second-placed Chelsea in FA Cup action this weekend, United can return to the top of the Premier League with victory over Hull.
United then follow that fixture with a visit to Arsenal on January 31.
"The nature of this league is that points are being dropped everywhere," Ferguson added. "If you win your games consistently, you're top of the league and that's the best place to be.
"Arsenal proved that recently, they've won their games and now they're top of the league, having been written off a few weeks ago. That's the nature of the league this season.
"But tomorrow is an opportunity for us to get that top position. Arsenal, for us, next Sunday is a massive game and then they play Chelsea the week after so it's interesting times now."