A subdued, under-capacity crowd watched as Erik Ten Hag’s side struggled without scoring.
Cristiano Ronaldo, making his debut in the Europa League and playing in his first season outside the Champions League since 2002, came the closest to scoring when his first-half header was ruled out for offside.
But it was a frustrating night for the Portuguese star, who started after four weeks on the bench. The forward made it clear earlier in the summer he wanted to leave United, citing his desire to play in the Champions League as the main reason for trying to force a move.
The home side – with six substitutions following their win over Arsenal at the weekend – controlled the first half but struggled to break up a well-organized Sociedad.
New signing Antony had saved a try before Ronaldo rose highest to meet a Diego Dalot cross.
Goalkeeper Álex Remiro couldn’t stop the ball, but he couldn’t stop the line, but the Red Devils’ celebration was cut short when the offside flag was raised.
Ronaldo threatened twice more with a header and a shot from outside the box before the Spaniards struck.
Lisandro Martínez, who came on for Dalot at half-time, blocked former Manchester City star David Silva’s shot but the ball bounced off his leg and onto his arm, resulting in referee Marco Di Bello giving the visitors a controversial penalty gave.
La Real’s summer signing Méndez made no mistake and put the Spanish club on the path to a first win on English soil in European competition.
Despite the famous victory, the game ended on a low note after the day’s news. All post-match interviews have also been canceled out of respect for the royal family.
The result leaves Ten Hag’s side third in Group E after Sheriff Tiraspol’s 3-0 win over Omonia Nicosia in the group’s other match.
The Dutchman will need to bounce back from defeat quickly as United prepare for a trip to Moldova to face Sheriff on September 15.