The Red Devils were unconvincing in Saturday's 2-1 win over West Ham United.
They will need to overcome a 1-0 deficit at the Nou Camp in order to reach the Champions League semi-finals.
De Gea said: "Of course it's difficult, but if you want to be a top team you have to play big Champions League games, and the weekend after it's the Premier League.
"So you have to be focused and ready for a difficult game again. I think we didn't play well [against West Ham], we have to improve a lot and now we have a big challenge on Tuesday.
"We will try to fight and win the game. We can go with more confidence after the three points.
"It's a big challenge, a special occasion, in Spain, against Barcelona, a Champions League night, so it's going to be an amazing game.
"We're happy with the win, but at the same time we have to improve a lot."