Madrid travel to the El Prat this having won their last seven matches in all competitions, but Aguirre has hinted that his side cannot afford to suffer a defeat this evening.
"They are the favourites and we must understand this. We must go to next week's game with options. Then we can go all out if need be," he told reporters at a press conference.
"By keeping it alive I mean not having a difference of two or three goals because then the tie is decided. We'll try to stay near them, both on the pitch and the scoreboard."
The second leg will take place at the Bernabeu next week.