The Catalan giants can ill-afford another slip-up in Group F after a 3-2 loss at Paris Saint-Germain last time out.
With the season's first Clasico coming just four days after the visit of the Eredivisie champions, coach Luis Enrique has already warned his players against taking their eye off the ball in midweek.
Those sentiments were echoed by Mascherano as the versatile Argentina international set his sights on success in Europe, with domestic ambitions on the back burner for now.
"The most important game is always the next one," he told the club's official website.
"If you win [the next game], that changes the next one.
"We need to win against Ajax and go into the next game [Clasico] with confidence."
Xavi added: "After the slip-up in Paris, we have to take three points."
Barca, who will be without defender Thomas Vermaelen (hamstring), beat APOEL 1-0 on match day one, but their loss in the French capital sees them second in the group - one point behind PSG.
Luis Enrique's men head into the game sitting top of the Spanish top flight and fresh from a 3-0 win over Eibar, in which striker Lionel Messi scored once to move to within two strikes of becoming La Liga's record goalscorer.
Draws against PSG and APOEL have Ajax reasonably placed after two matches and stalemates have been a common theme for Frank de Boer's side in recent times.
Ajax have had to share the spoils in their last three clashes - most recently a 1-1 draw against Twente on Saturday.
Kolbeinn Sigthorsson netted a 71st-minute equaliser as the four-time defending Eredivisie champions fell four points behind leaders PSV.
The two sides played each other in the group stages last term as Messi scored a hat-trick in Barcelona's 4-0 home win.
But Ajax won their home clash 2-1, with Thulani Serero, a doubt for this game with a hamstring injury, and Danny Hoesen both scoring in the first half.
De Boer will be looking to get one over on the club he spent four years at as a player between 1999 and 2003.
The Dutchman won the league title at Camp Nou in his first season.
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