Manchester United midfielder Anderson is out for the rest of the season after rupturing his cruciate ligament during the win over West Ham.
The Brazilian was making his first appearance since the Carling Cup tie at Manchester City last month when he crumpled in agony during the first half at Old Trafford.
It was immediately obvious there was a serious problem and although the former Porto star tried to continue, he was forced to abandon that plan within minutes.
Anderson went for a scan on Wednesday, with Sir Alex Ferguson confirming that the player will fly to Portugal in a fortnight for an operation.
"The real bad news is that Anderson has a cruciate knee injury," said the United boss.
"He will be out for the rest of the season. It is a bad blow."
Anderson's Old Trafford career had been under a cloud, with the midfielder unhappy at the amount of games he was missing recently and the impact it would have on his chances of featuring in the World Cup for Brazil.
But Ferguson had the player in mind for a starting spot in the Champions League tie with AC Milan at Old Trafford on March 10, when Michael Carrick will be suspended.
Now, both options are ruled out, with Ferguson hoping Anderson is back for the start of next season.
"He was looking forward to the match on Tuesday and he was showing great energy to do well," reflected Ferguson.
"It is a bit of a blow to us, particularly with the AC Milan game coming up because Michael Carrick is suspended.
"It is strange, when you see cruciate injuries it is usually something simple. He stretched out for the ball and his knee just popped.
"We have assessed the situation and know he needs the operation.
"That will be done in two weeks' time in Portugal once the swelling has gone down. Sometimes we have sent them to Dr Steadman in Colorado. In this instance he is comfortable with the surgeon he has dealt with in Portugal in the past.
"He will be out for six months and hopefully that should have him about right for the new season."
