Coach Jagoba Arrasate has acknowledged that fourth spot and a subsequent Champions League place is the ultimate aim for his team, currently sixth in the league table, but set toppling Villarreal as the immediate aim.
"If we are to reach fourth spot we must reach fifth first of all," he told reporters.
"Villarreal's result is good for us, as long as we keep winning, but it also proves that it's getting harder and harder to win, and should remind us that we cannot be complacent and think we're going to beat Rayo just because we're playing at Anoeta.
"In order to think about that [overtaking Villarreal], first of all we must win tomorrow."
La Real's ambitions of making the top four for a second straight season were dealt a blow last weekend when they were beaten 1-0 at Sevilla.
Arrasate explained his team would revert to a more attacking 4-3-3 system against Rayo than they had employed against Sevilla and in their 3-1 win over Barcelona a fortnight ago, meaning that striker Imanol Agirretxe can expect to return to the starting line-up.
The coach added it was unlikely that Claudio Bravo would be fit to play a week after suffering a hamstring injury, and ruled out Jon Ansotegi and Diego Ifran returning on time from their respective injuries.
Academy product Enaut Zubikarai is therefore expected to start in goal.
Rayo earned a surprise victory at home to Valencia last week and are under pressure to take all three points from Anoeta, after fellow strugglers Valladolid and Granada both won on Saturday.
But coach Paco Jemez insisted his team were not bothered by the results of other teams.
"We don't care what the rest do," he told a press conference.
"The fact that the other teams have won doesn't affect our team spirit, we depend on our own results and we cannot worry about what our rivals do. We'd be committing a huge error if our morale was dictated by other teams."
The Madrid side's only fresh absentee is captain Roberto Trashorras, who is suspended.