Manchester United manager Sir Alex Ferguson has confirmed midfielder Anderson will be out of action for between four and five weeks with a hamstring injury.
It represents a further injury blow to the Brazilian, who has endured an injury-plagued season.
The 23-year-old began the campaign in fine form but has only played five times for the Red Devils since sustaining a knee injury in November.
He made his comeback against Athletic Bilbao in the Europa League last 32 first leg at Old Trafford last week but later complained of a pain in his thigh and a tight hamstring.
Speaking ahead of his side's trip to Bilbao for the return game, Ferguson confirmed the midfielder had suffered a hamstring injury and is set to spend at least a month on the sidelines.
"Anderson could be out for four or five weeks due to his hamstring," Ferguson told the club's official website.
"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 of where we stand with him [in terms of recovery]."
The Scot also confirmed that Rio Ferdinand would start alongside Jonny Evans in defence, with Phil Jones not travelling due to Flu and Chris Smalling only fit enough to make the bench.
United trail 3-2 following an action packed first leg at Old Trafford last Thursday and Ferguson acknowledged his side are up against it in the Basque Country
However, the 70-year-old remains confident that his side can overcome the deficit and progress through to the last 16.
"It's a difficult tie for us," he added. "Bilbao have a tremendous advantage after winning at Old Trafford. It's a challenge but not one that is beyond us.
"Our record away from home helps us - we've done very well. We're going to need a good performance."