Barça attacked from the outset in the first leg at Camp Nou but were nonetheless held to a 0-0 draw, giving United a narrow advantage.
Rijkaard, however, remains positive in the face of a trip to the north of England.
He said, "It is a difficult ground and a strong team, but I think that we can get through."
However, he recognises that it will not be easy.
The boss continued, "United are a team that have no fear. They play how they know, with a good attitude, with a lot of individual talent and without complicating things.
"On Tuesday I am sure they will come out more to attack than at the Nou Camp and we must take advantage of our chances. The keys to the tie? To be aggressive and put them under a lot of pressure so as to make life difficult for them.
"To defend the result is not our style. We need to worry only about our play."
On the subject of playstyle, he added, "I think that we have always maintained our style of play wherever we have been, with only one exception, which was when we went to play at Chelsea's ground," Rijkaard said, recalling the 1-0 defeat at Stamford Bridge in October 2006.
"There we played much more defensively than we are used to because at that point Chelsea were a very well-armed, organised and hard-working team, who were very dangerous on the counter-attack.
"But 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 by putting them under pressure.
"My job? The club have always helped me a lot but we need to see what is best for club.
"I can imagine the logical anger of the fans [if we lose] but we are going to give everything to bring the supporters some joy."
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