Ferguson: Rio return date still uncertain
The England international has missed United's last 13 games and is not in contention for Sunday's Premier League trip to struggling Hull City.
"I don't think he's too far away but it's difficult to put a time limit on back injuries," Ferguson told United's website.
"We gave him an injection just over two weeks ago and we had to let that settle down for a week.
"He's since been carrying on with his gym work and we're hoping the pain will go away.
"He's not shown his frustration - he's quite a composed, laid-back lad, but I'm sure he's feeling it."
United's defence has been decimated by injuries in recent weeks with midfielders Michael Carrick and Darren Fletcher being forced to play at the back in the absence of Ferdinand, Nemanja Vidic, Jonny Evans, Wes Brown, John O'Shea and Gary Neville.
Champions United were hammered 3-0 at Fulham last week, their second league defeat in three games, leaving them four points behind leaders Chelsea at Christmas.