"We had our chances to make it 2-0 in Madrid," he told Barca TV. "In the return leg we have to be as effective as possible because they are very effective. We have to be on target in front of goal if we want to get to the final.
"It's vital to score, we cannot go into the match and wait to see what happens. In last year's Cup Clasico return leg - we had a good result from the away leg - and we were nearly eliminated from the competition.
"Madrid are very dangerous and in order to go through we'll have to play the best football we're capable of."
Fabregas has scored six goals in 21 La Liga appearances for Barcelona so far this season.