Sir Alex Ferguson is expecting Athletic Bilbao to provide his Manchester United side with a stern test over the next couple of weeks.
United play host to their Spanish opposition tomorrow evening, before travelling to San Mames seven days later for the return leg.
The Red Devils are favourites to secure progression through to the next round but Ferguson is wary of the threat posed by Bilbao.
"It's a very difficult tie," Ferguson told reporters. "All the work Bilbao have done has been very impressive. They're a very progressive team in Spain at the moment and it's going to be very difficult for us.
"We need to waken up, there's not question about that. Some of our European form has been disappointing.
"But it's an opportunity to progress and we want to take that opportunity, although it's going to need a very good performance."
Ferguson is expected to be without the services of Paul Scholes and Michael Owen tomorrow night.
