An Aritz Aduriz hat-trick and a San Jose goal saw the European champions demolished by their Basque opponents at San Mames in a game which has put Barca's chase for the 2015 sextuple in doubt.
However, Mascherano believes his side have what it takes to come back from the defeat and still win the title, but maintains they must be more focused defensively.
"We didn't play well, that goes without saying, but I think the scoreline was worse than we deserved in the first leg," the Argentine told reporters.
"They had maybe six or seven chances and scored four goals, including a penalty. We had five or six chances but didn't score.
"I think it was isolated errors that punished us more than we deserved. We need to improve and minimise errors but we don't need to change everything.
"This team is reliable and this season we will get better, but now we need to think about how to turn this game around.
"We're excited about this. It's a unique challenge. We've got the tools to turn it around. In many occasions in the past this team has won games by a lot of goals, so we have to try to do this on Monday.
"We're not trying to be arrogant, they are a tough opponent, but we believe in our abilities. I'm sure we will be trying every way possible to win the game. It's essential that we don't let them score.
"Football's a game in which you pay for your mistakes. Sometimes you make mistakes yourself or the opponent forces you into them."
Mascherano - who can play in defence or midfield - maintains there is no need for Barca to completely change their style heading into the clash at Camp Nou.
"It's not a question of going mad and doing things we've never done before. We have to play the way we do naturally, that's how we go about winning it.
"I think we played against a team that were tough for us last season too. They are a team that challenge for every ball and make you uncomfortable. They're powerful in the air and it's always hard to play against them."