Ferguson unsure if Rio will return
"My concern is getting Rio back for some part of this season and hopefully we can do that," Ferguson told a news conference.
England captain Ferdinand suffered a calf injury in the warm-up before United's 2-1 league defeat by Wolverhampton Wanderers on February 5.
Central defender Nemanja Vidic will also miss Saturday's Premier League game against Bolton Wanderers along with John O'Shea and Rafael da Silva, who both suffered hamstring injuries during Tuesday's Champions League win over Olympique Marseille.
"We're having a bad spell," Ferguson said. "Only a few weeks ago we had every defender fit and looking great."
United are clinging on to a three-point lead over Arsenal at the top of the Premier League, having played a game more, and they are through to the Champions League quarter-finals and FA Cup semi-finals.
Defender Jonny Evans has returned to training but Ferguson is again likely to play Chris Smalling and Wes Brown in central defence against Bolton.
"I'll hopefully have Jonny Evans fit for Saturday, although it's a bit of a risk as he's been out for so long.
"We've also got the bonus of Wes being able to play right-back and centre-back and Chris Smalling coming to the club. He's been brilliant."