The Catalan outfit played for more than one hour against ten men after Atletico had goalscorer Fernando Torres sent off in the first period and a Luis Suarez double saw the reigning champions triumph 2-1.
Mascherano has said that his team "had chances to score some more goals", but insists that winning the match was the most important thing ahead of the return in Madrid next week.
"It was important to win. We had chances to score some more goals, but it is important to win at home against a team that is very strong. We know that we have to go to Madrid and do our work and we will try to do that," Mascherano told BT Sport. "I think we had some clear chances to score some more goals, but the main target was to win and we did that."
Mascherano refused to comment on the incident that saw Torres receive his marching orders.