Los Che have picked up one point from their last two games and are five points behind Sevilla, who occupy the final European spot in the Primera Division.
"There are fewer and fewer points to play for and, if we want to get into Europe, we have to get all the points we can," Pizzi told a press conference.
Valencia's positive run in the Europa League has been one silver lining in an otherwise disappointing season for the club, and they head to San Sebastian just three days after returning from Bulgaria, having eased to a 3-0 win at Ludogorets that puts them on their way to the quarter-finals of the competition.
However, Pizzi dismissed suggestions his team were focusing more in Europe than on their league games.
"There's no difference between how we're playing in the league and in Europe," he said. "Our aim is always to play to our limits and, if we can be competitive, then it's very difficult for things to go wrong for us.
"We have to make sure we are competitive in every game so we can get good results."
Valencia will be without centre-back Jeremy Mathieu and forward Paco Alcacer, both of whom are suspended. Pizzi admitted both were "important players" to his side but insisted: "We cannot use that as an excuse. We've got a big squad."
Sociedad also need a big result to prevent their campaign from derailing, having suffered defeat at Sevilla and then lost 3-2 at home to Rayo Vallecano, despite going ahead twice in the match.
"Over the week, we've spoken about everything we did badly at Rayo so that we don't make the same mistakes tomorrow," coach Jagoba Arrasate told reporters.
Seven points separate Sociedad and Valencia in the table, and a home win would see the Basques take an important step towards finishing above Los Che for the second year in a row.
La Real pipped Valencia to the fourth Champions League spot on the last day of last season, and the new-found rivalry will take a new twist on Sunday, with Sergio Canales facing his old club for the first time since moving from Valencia to San Sebastian in January.
The forward admitted during the week he would celebrate if he managed to score against his former employers, prompting Pizzi to respond with a message of indifference.
"I don't think it's very controversial," he said. "We are not upset that Canales no longer plays for us, but it won't motivate us any more either."
Sociedad have no fresh injury concerns after the defeat to Rayo but are hopeful that Claudio Bravo will recover from a muscle injury in time to face Valencia, his stand-in Enaut Zubikarai having made a costly error last weekend which led to Rayo's second equaliser.