Manchester United will be without injured midfielder Anderson for a key part of their crucial Premier League title run-in.
United boss Sir Alex Ferguson has confirmed the 23-year-old will miss up to five weeks with a hamstring injury.
The Brazilian, who has made 16 appearances for the Red Devils this season, has been ruled out of United's Europa League tie at Athletic Bilbao.
He could also miss up to six vital league games for the leaders.
"Some hamstrings are relatively straightforward but we have to wait and see how he is in the next week or so to get a better idea," said Ferguson.
United, who are one point clear of neighbours Manchester City with 10 games remaining, returned to the top of the league with victory over West Brom on Sunday.
But their assault on the title has been largely made without Anderson, who has been sidelined for the majority of the winter with a knee injury.
He only returned to Ferguson's matchday squad for the recent 3-1 win at Tottenham before making his first appearance for two months as a 61st minute substitute in last week's first-leg defeat by Bilbao.
United are looking to overturn a 3-2 deficit in Bilbao on Thursday to progress into the last eight of the Europa League.
Anderson is also set to miss league games against Wolves and Fulham later this month, before United face Queens Park Rangers, Wigan and Aston Villa before the middle of April.