Anderson ruled out until February
There was better news for striker Wayne Rooney, who missed Tuesday's 2-2 draw at home to Benfica in the Champions League with a hip injury, as he will be available for Saturday's Premier League match at home to Newcastle United.
"We've sent him [Anderson] back to Portugal to see the specialist who operated on his knee last time. That's a bit of a blow, we didn't expect that one," Ferguson told a news conference.
"Rooney should be OK for tomorrow, he trained yesterday."
Ferguson was already having to manage without injured midfielder Tom Cleverley before the extent of Anderson's injury was known but the Scot was not overly concerned.
"We're not as bad as people think we are in midfield," he said. "We have Ryan Giggs, Michael Carrick and Darren Fletcher, and if we get Tom Cleverley back for Christmas that'll be a bonus. We'll be okay, we have a strong squad."
Asked whether he was worried enough about the midfield to sign someone in the next transfer window, Ferguson replied: "It's dead easy to say you'll go out and buy players but it's not easy to buy players who are good enough for Manchester United, especially in January."
Ferguson's upbeat outlook is at odds with some below-par performances.
His side lie second in the league table, five points behind Manchester City. While they had strung together a run of five wins with clean sheets since last month's 6-1 demolition by their city neighbours, that came to a grinding halt this week.
Two defensive blunders allowed Benfica to leave Old Trafford with a draw and a place in the Champions League last 16, while United's bid to reach the knockout stage has been left hanging in the balance until next month's final round of matches.
Captain Nemanja Vidic, who has played a key part in tightening up the back four, is available to face Newcastle having missed the Benfica game through suspension while fellow defenders Chris Smalling and Rafael are fit again.
Visitors Newcastle lost their unbeaten record last weekend in a 3-1 defeat at Manchester City and Ferguson knows they will be tough opponents.
"They're all working like Trojans up there," he said. "Newcastle aren't doing much wrong at the moment and the quality of the league is improving - you can throw a net over the top five or six."
Ferguson was bracing himself for a challenging month, with six league games in December, as he tries to make up ground on free-scoring leaders City.
"December is an arduous month for everyone," he said. "But you have a chance in this league if you're still in the ball park by New Year. And the way this club is organised to perform in the second half of the season helps us."