Brazil international Anderson was ruled out of action until February because of a knee injury, further limiting the team's midfield options at a crucial time of the season.
''He's been sent to Portugal to see a specialist who operated on him before. Not good news,'' United manager Alex Ferguson said Friday.
With Tom Cleverley not expected back before Christmas because of an ankle problem, Ferguson is without the players who were his first-choice central-midfield pairing at the start of the season.
The Scottish coach will rely on Darren Fletcher, Michael Carrick and veteran Ryan Giggs to plug the gaps in that position.
United must avoid defeat against FC Basel next month to qualify for the last 16 of the Champions League and also has a hectic Premier League schedule in the run-up to the New Year as it attempts to chase down Manchester City, which holds a five-point lead.
Anderson, who last played for United in the Champions League win against Romanian team Otelul Galati on Nov. 2, started the season well, scoring rare goals against Tottenham and Norwich.
