Barcelona coach Xavi Hernandez urged his team to improve their finishing against Atletico Madrid on Sunday in La Liga, where they will be without top goalscorer Robert Lewandowski.
The Polish forward has 13 goals in the top flight but will miss the first of three matches suspended following a red card against Osasuna in November before the World Cup break.
After Barcelona appealed against his ban he was allowed to feature in the 1-1 draw against Espanyol last weekend but his punishment was later upheld.
"Against Espanyol we lacked aggression in their box and we lacked efficiency," Xavi told a news conference on Saturday.
"It's a mental question. In recent games we have played well and generated many chances, but we lacked the strength to show our superiority on the scoreboard.
"Both Espanyol and Intercity (whom Barcelona beat 4-3 in the Spanish Cup) made life hard for us as we didn't take all the chances we had."
Barcelona lead the league on goal difference ahead of Real Madrid, but the reigning champions can move ahead with a win at Villarreal on Saturday.
Atletico, fourth, have not been at their best this season but Xavi anticipated a tricky encounter.
"(Simeone's) teams are defensively very well drilled," said Xavi.
"They've been changing systems and it's hard to predict how his team will play.
"Atletico have virtually no defensive holes, we have to take advantage of our chances as we won't have many."
Barcelona's coach must decide between Ansu Fati, Ferran Torres and Memphis Depay for Lewandowski's replacement at the Metropolitano.
"We will play with a No 9, you will see who it is tomorrow," added Xavi, keeping his cards tight to his chest.
"We have tried out different profiles and tomorrow we will take the decision."