The match will take place a little over 24 hours after the Red Devils are in action against Atletico Madrid in Norway, while Rayo will enter the contest off the back of a 2-1 victory over Leganes in a friendly.
Match preview
Man United announced their squad list for Saturday's game with Atletico on Friday afternoon, with a strong group travelling to Norway for the contest against Diego Simeone's side, meaning that the game with Rayo will provide opportunities for a number of players in need of minutes.
The Red Devils have had a successful pre-season until this point, opening their preparations for the new campaign with a 4-0 victory over Liverpool in Thailand on July 12.
Erik ten Hag's side then beat Melbourne Victory and Crystal Palace in Melbourne before playing out a 2-2 draw with Aston Villa in Perth before heading back to Manchester last weekend.
The club also took part in a behind-closed-doors fixture with Wrexham on Wednesday, recording a 4-1 victory, with Christian Eriksen and Lisandro Martinez both making their unofficial debuts for the club; at the time of writing, there has been three new signings this summer, with Tyrell Malacia also arriving from Feyenoord.
This match will represent Ten Hag's first home fixture in charge of Man United, and the new Premier League campaign is now just around the corner, with Brighton & Hove Albion visiting Manchester on August 7, and there will be pressure on the Dutchman to make the perfect start.
Rayo, meanwhile, are building towards their La Liga opener against Barcelona on August 13, with the team already taking part in four pre-season matches.
Andoni Iraola's side opened their preparations for the new season with a 3-0 defeat at Freiburg on July 16 before recording a 2-0 victory over Sheffield Wednesday four days later.
Rayo then played out a 2-2 draw with Birmingham City on July 23 before beating Leganes 2-1 last time out, but it has been a relatively quiet transfer window for the club thus far, only bringing in three players, with Unai Lopez, Salvi Sanchez and Diego Lopez arriving on free transfers.
There has been talk over a return to the club for Diego Costa, although the move is yet to go through, so the former Chelsea forward will not be involved at Old Trafford this weekend.
Los Franjirrojos enjoyed a successful 2021-22 campaign on their return to the top flight, finishing in 12th position in the table, which represented their best performance in La Liga since 2015.
Manchester United friendlies form:
Manchester United form (all competitions):
Rayo Vallecano friendlies form:
Rayo Vallecano form (all competitions):
Team News
The bulk of Man United's first-team regulars have travelled to Atletico, but there were some notable absentees from the squad, including Cristiano Ronaldo.
The Portugal international is allegedly still making it clear to the Red Devils that he wants to leave this summer, but there is a chance that he could be involved in this contest.
Raphael Varane, Aaron Wan-Bissaka, Eric Bailly and Luke Shaw could feature at the back, while there are likely to be spots in the final third of the field for Amad Diallo and Alejandro Garnacho.
Charlie Savage and Zidane Iqbal both impressed during the club's pre-season tour and could also feature, while there might be a spot between the sticks for Nathan Bishop.
As for Rayo, former Man United striker Radamel Falcao is likely to feature for the Spanish outfit.
Falcao represented the Red Devils on loan during the 2014-15 campaign, scoring four times in 29 appearances for the club in all competitions.
Veteran goalkeeper Diego Lopez could start between the sticks, while fellow new signings Unai Lopez and Sanchez will be hoping to feature, with head coach Iraola expected to make full use of his squad.
Rayo also have another ex-Man United attacker in their squad in the shape of Bebe, with the Portuguese turning out for the club between 2010 and 2014, making just seven appearances for the English giants.
Manchester United possible starting lineup:
Bishop; Wan-Bissaka, Varane, Bailly, Shaw; Savage, Iqbal, Hannibal; Amad, Chong, Garnacho
Rayo Vallecano possible starting lineup:
D Lopez; Garcia, Balliu, Catena, Hernandez; U Lopez, Valentin, Ciss; Trejo; Palazon, Falcao
We say: Manchester United 2-2 Rayo Vallecano
A number of youngsters will be given chances to impress for Man United, and it should be an open and entertaining contest against a team that like to get the ball down and play. We can see goals at Old Trafford on Sunday afternoon, with the spoils potentially being shared in Ten Hag's first home fixture at the helm.