The Basques currently occupy the final Champions League spot in the La Liga table but are just two points ahead of Valencia, who will unseat them in fourth if they can take all three points.
"This is the closest game we've had to a final until now," Valverde told a press conference.
"Real Sociedad are on an unbelievable run and these three points mean a lot. If we win we'll go ahead of them, if we lose we'll be very far behind, and if we draw it all stays the same. It's a decisive game."
Valencia go into the match after thumping fellow European pretenders Malaga 5-1 last weekend, scoring four goals in the space of six minutes.
Although Real Sociedad are renowned for their attacking football, Valverde insisted his team would not hold back and would try and put pressure on the hosts from the off.
"That's the way we like it, we assume the pressure, and that's how I love to play. When we have that pressure we always play well."
Elsewhere on Sunday, Malaga entertain Getafe, Granada travel to Espanyol while Sevilla host Real Valladolid.
Monday's only game sees Real Betis take on Deportivo La Coruna at the Estadio Benito Villamarin.
Leaders Barcelona will have to wait another week before they can seal the title after being held to a 2-2 draw by Athletic Bilbao on Saturday. The Catalans were headed for victory after second-half goals from Lionel Messi and Alexis Sanchez cancelled out Markel Susaeta's opener, but Ander Herrera netted in the final minute to earn a share of the spoils for the Basque outfit.
This also means that Real Madrid retain the slimmest of hopes of retaining the La Liga crown after they beat derby rivals Atletico Madrid 2-1 at the Vicente Calderon. The Rojiblancos had taken a fourth-minute lead through a Radamel Falcao strike, but Juanfran's own goal Real back on level terms, before Angel Di Maria netted in the 62nd minute to hand the visitors all three points.
Elsewhere, Augusto Fernandez scored the only goal of the game to help relegation-threatened Celta Vigo to a vital 1-0 win at Levante, while Real Zaragoza beat Real Mallorca 3-2 in a relegation six-pointer.
Meanwhile on Friday, Rayo Vallecano missed the chance to close in on a European spot as they had to come from behind to draw 2-2 with relegation-threatened Osasuna in the La Liga.
Sunday's La Liga Fixtures
Monday's La Liga Fixtures
Real Betis vs Deportivo La Coruna
Saturday's La Liga Results
Friday's La Liga Results