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."
However, the coach does not think Real Sociedad will have a conservative game plan either.
He said: "I don't think they will let us have the initiative. They have decisive players in the final third and they are at home and they will try and make home advantage count."
Real Sociedad goalkeeper Claudio Brave also expects an end-to-end match.
"The two teams both have clear ideas of how to compete and it's going to be a very good game," he told reporters.
"There is a difference in pressure each team applies, that's obvious, but at this stage of the season we both need to pick up points because we both want to finish fourth."
The Chilean stopper acknowledged the game was of great importance but said his team must not be overwhelmed by the pressure, because there will still be five games remaining until the end of the season.
He said: "It's important but so were the games against Rayo Vallecano and Osasuna.
"Every game we have left is like a final, but we need to approach each game in a relaxed way. We can't put too much pressure on ourselves."
He also emphasised how proud the team were of their achievements this season and said they were determined for it to get even better.
"It's a dream to be where we are at the moment but we need to keep going because there are still six games left for us to play and finish the job," said Brave.
"Valencia are a very good team and we need to keep working as hard as we have been to keep the dream alive."
Sociedad have no fresh injury concerns but will be without the suspended Antoine Griezmann, one of their stand-out performers this term.
Valencia, meanwhile, will be missing forward Sergio Canales and defender Adil Rami, who are both injured.