Holders Barca have a slight advantage for Tuesday's semi-final second leg at the Nou Camp after securing a 1-1 draw at Real's Bernabeu stadium in last month's first leg.
However, they showed a lack of ideas in attack in losing 2-0 at an ultra-defensive AC Milan in last week's Champions League last 16 first leg and again appeared vulnerable at the back in coming from a goal down to rescue a 2-1 La Liga win at home to Sevilla on Saturday.
It was the 11th match in a row in all competitions in which the runaway La Liga leaders failed to keep a clean sheet.
"We have seen that some of the goals have been conceded through our own fault and we will try to correct that," Roura, standing in for Tito Vilanova while he recovers from throat surgery, told a news conference.
With Real forward Cristiano Ronaldo firing on all cylinders, a tight defence is particularly important for Barca, who play at the Bernabeu in La Liga on Saturday.
Before the first leg, the Portuguese had scored at least once in his last six "Clasicos" and headed the goal that gave Real a 1-0 victory over their arch rivals in the 2011 cup final.
Fabregas, who netted Barca's goal in the first leg before Raphael Varane grabbed a late equaliser, said he and his team mates will need to be as ruthless as Real in taking their chances.
"We have to be as effective as possible in the return game because they are," he told Barca TV on Monday.
"We have to be on good form in front of goal if we want to get through," added the former Arsenal captain.
"It's crucial to score and not to sit back and wait and see what happens.
"Madrid are very dangerous and to qualify for the final we have to play the best football we are capable of."
