Sir Alex Ferguson has confirmed midfielder Anderson will be out of action for between four and five weeks with a hamstring injury.
Injury-plagued Anderson made his return after two months out in the 3-2 first leg defeat against Bilbao last week, but complained about a tight hamstring and pain in his thigh after the match.
Now the former Porto midfielder faces a long lay-off, with manager Sir Alex Ferguson eager to avoid exacerbating the problem. United were overrun in midfield by Bilbao at Old Trafford and Ferguson had hoped to use Anderson to shore up his side as he seeks a result at the San Mames.
However, there was some good news for the United manager, as goalkeeper Anders Lindegaard returned to full training after damaging ankle ligaments last month.