The Brazilian midfielder took a knock to the knee in Saturday's clash with Crawley and scans have revealed cartilage damage.
Yesterday Anderson left the Carrington training ground on crutches.
United are still hopeful he could avoid surgery but have resigned themselves to being without the 22-year-old until mid-April.
And that will come as a huge blow as United are short in midfield with Antonio Valencia, Owen Hargreaves and Ji-Sung Park all still crocked.
But there was some good news as Patrice Evra penned a new 3½-year deal at Old Trafford.
SunSport told you first back on October 26 that the full-back was ready to commit his long-term future to the Red Devils.
Spanish giants Real Madrid and Barcelona were eyeing a swoop.
But Evra, 29, has ended any doubts by agreeing a £90,000-a-week contract which keeps him at United until 2014.
Evra said: "I've won a lot in the last five years, but I want to win more and I know that's the mentality of everyone here."
Manager Alex Ferguson added: "His experience and talent will be increasingly important to us."
Michael Carrick and Dimitar Berbatov are also close to extending deals.
I thought why not get leighton baines.
Just when Anderson showed some promise, this thing happens again. It's a vicious cycle. Will United sign Henderson or even Rodwell this summer? Can recall Cleverley or not ar?
cannot rely on this fellow lar. I hope fergie go out buy rodwell, henderson, Sneider or even Rossi
Manchester United boss Sir Alex Ferguson has cooled injury fears over Anderson, revealing that the midfielder could only miss two weeks of action.
Early reports had suggested that Anderson's knee problem, sustained the FA Cup win over Crawley Town, would sideline him for up to two months.
"Anderson got injured in the first half on Saturday," Ferguson added. "We have sent him to Portugal to see a specialist he has dealt with over there. That will be two weeks, three at the most."
Anderson is one of five United players who will miss the Marseille clash in the Champions League on Wednesday evening, and Ferguson expects all bar Michael Owen to be short-term absentees.
"Ryan Giggs should be okay for the Wigan game on Saturday, while Rio Ferdinand and Jonny Evans should be back for the Chelsea game next week," he said.
Meanwhile, Ferguson said that midfielder Antonio Valencia, who has been out of action since breaking his ankle in September, is in line to make a first-team return for the FA Cup quarter-final clash against Arsenal or Leyton Orient.
Heng ar, 2 weeks not 2 months.