It was reported on Friday morning that Rio Ferdinand - who had been ruled out for six weeks - was ahead of schedule. Ferguson has confirmed that Ferdinand could be available for the game against Arsenal next weekend, but it has emerged that Vidic now faces a lengthy absence.
"Last week we thought Rio's hamstring would have him out for six weeks and with Vidic, we thought it was just a nerve problem," Ferguson said. "That's reversed itself.
"Vidic has got a calf muscle injury, which means he'll be four to five weeks. It's longer than we thought. Meanwhile, Rio's injury isn't as bad as we thought and he could be ready for our game against Arsenal. I don't think he'll be ready for Monday. It's amazing how circumstances can change in a few days."
United are currently preparing for a difficult spell of fixtures in which they host Tottenham on Monday and Arsenal the following Sunday before a trip to Bolton and then a home game against Chelsea.
Rafael had already been ruled out for ten weeks with a shoulder injury and Ferguson said: "Rafael has had an operation this week and will be out for ten weeks. It was a freak injury really. He won a tackle [in training], stumbled and fell on his shoulder."
"Patrice is back which gives us experience," he said. "Chicharito is back training and doing very well as is Valencia, who will play in the reserves next week."
Asked when Hernandez will return to the first-team, Ferguson said: "It depends. From a medical point of view, considering he had a concussion before, we have to decide when is the best time to bring him back.
"Darren Fletcher had a game the other night, so he's back and doing well. He'll play maybe one more game in the reserves and then should be ready for the first team, so there are a few there, but they're all on the mend, we hope."
Catch action from every Barclays Premier League match on your mobile for FREE on the enhanced ESPN Goals app (UK only).