The visitors currently sit 19th in the table, four points from the safety of 17th position, while Rayo are 11th, just two points behind seventh-placed Girona in the race for a Europa Conference League spot.
Match preview
Rayo have never finished higher than eighth in Spain's top flight, so they have the opportunity to create some history this season, with a European spot in the table well within their grasp at this stage.
Andoni Iraola's side will enter the next set of fixtures in 11th position in the table, just two points behind seventh-placed Girona, who are currently in a spot which would see them qualify for the Europa Conference League playoff round for the 2023-24 campaign.
Rayo's recent home form has been excellent, winning each of their last three in front of their own fans, including a 2-1 success over the champions Barcelona at the end of last month.
The capital outfit will enter this match off the back of a defeat, though, suffering a 3-1 loss to Real Betis on Monday, which has left them in 11th spot in the division on 46 points.
Los Franjirrojos have actually won their last three top-light matches against Espanyol without conceding, including a 2-0 victory in the reverse game between the two sides back in August.
Espanyol would have loved nothing more than to prevent Catalan rivals Barcelona winning the title last weekend, but they suffered a 4-2 home loss to Xavi's side, which further increased their relegation fears.
There is no getting away from the fact that it has been a tough campaign for Luis Garcia's team, who are currently 19th in the table on 31 points, four points behind 17th-placed Real Sociedad with four games left.
Espanyol are certainly capable of moving out of the bottom three over the next couple of games, but another poor result here would leave them in a vulnerable position ahead of a home match against Atletico Madrid.
The Catalan outfit were 1-0 winners over Getafe at the end of April but have since lost their last two to Sevilla and Barcelona, making it three defeats from their last four games at this level of football.
The White and Blues finished 14th in La Liga last season on their return to the top flight, but they are in serious danger of dropping down to the second tier for next term, which would be a surprise considering their quality in the forward areas, with Joselu on 15 league goals for the campaign and Martin Braithwaite on nine.
Rayo Vallecano La Liga form:
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Espanyol La Liga form:
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Team News
Rayo will again be without the services of Esteban Saveljich due to injury, while Mario Suarez and Oscar Valentin are both suspended for the home side this weekend.
As a result, there will have to be at least one change to the side that started against Real Betis, with Pathe Ciss expected to come in for a start in central midfield.
There is also expected to be an alteration in the final third of the field, though, with Sergio Camello likely to be given the nod over Raul de Tomas, who is set to drop down to the bench.
As for Espanyol, Jose Gragera remains on the sidelines with a muscular issue, but the visitors are otherwise in strong shape heading into this match.
As mentioned, in Joselu and Braithwaite, Espanyol boast two regular goalscorers at this level, and the pair will continue in the final third of the field on Sunday.
Head coach Garcia is expected to change his formation to a 4-3-3, which would open up a spot in an attacking area for Javi Puado, with Sergi Gomez likely to drop out of the XI from the Barcelona game.
Rayo Vallecano possible starting lineup:
Dimitrievski; Balliu, Mumin, Catena, F Garcia; Comesana, Ciss; Palazon, Trejo, A Garcia; Camello
Espanyol possible starting lineup:
Pacheco; Gil, Montes, Cabrera, Olivan; Melamed, Darder, D Suarez; Braithwaite, Joselu, Puado
We say: Rayo Vallecano 2-1 Espanyol
This is a huge match for both sides, and it will be fascinating to see what develops on Sunday. Espanyol desperately need to win, but they have only been victorious three times on their travels this term, while Rayo have been largely strong at home, so we are expecting the hosts to triumph in this contest.