كتب : Omar AbdelAziz
Both sides had a disappointing weekend domestically, as United lost 2-1 at Chelsea, while Barca were outclassed by Deportivo La Coruna.
However, the Catalans are in a more critical situation as they have one last chance to save their miserable season.
Frank Rijkaard's team have lately managed only one win in nine games in the Primera Liga, but the Dutchman calls for self-belief against the Premiership champions.
"You've got to believe in yourself, fight hard, work hard, show our quality out on the field. That will give us a good chance," he said.
"Against Manchester United we have to play strong. To defend is not the correct course of action in this case. We have to play with aggression and put them under pressure."
"Psychologically it could be difficult and it could be interesting."
Barca welcome back skipper Carles Puyol, who was suspended in the first-leg goalless draw, but Rafael Marquez misses out.
On the other hand, Sir Alex Ferguson is sweating on the fitness of two key men, center-half Nemanja Vidic and striker Wayne Rooney.
"We think both could make it but I can’t be certain," the United boss said.
Ferguson rested a handful of players in the Stamford Bridge defeat in an attempt to have a fresh squad against Barca.
"Playing Wednesday night after a lunchtime kick-off there is no way I could have played the same teams," he said.
"So I had to decide where I could get the best freshness and give my team the best chance of a European final because that is the best incentive for any player.
"Time and time again I said this club should have had more success in Europe and I wanted to give them the very best chance and it was a very easy team to pick on Saturday.
"We are disappointed in the manner in which we lost. That is normal, but with the incentive of a big game you put that behind you quickly and the intensity of the training session reflected that."