A subdued, below-capacity crowd watched as Erik Ten Hag’s side toiled without scoring.
Cristiano Ronaldo, making his first ever appearance in the Europa League and playing in his first season outside the Champions League since 2002, came closest to scoring when his first-half, headed effort was ruled out for offside.
But it was a frustrating night for the Portuguese star who was in the starting line-up following a four week period starting on the bench. The forward made it clear earlier in the summer that 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 — featuring six changes from its win over Arsenal at the weekend — controlled the first half but struggled to breakdown a well organized Sociedad.
New signing Antony had an attempt saved before Ronaldo rose highest to meet a Diego Dalot cross.
Goal keeper Álex Remiro got a hand to it but was unable to stop the ball crossing the line, but the Red Devils’ celebrations were cut short when the offside flag was raised.
Ronaldo threatened twice more with a header and a strike from outside the box before the Spanish side struck.
Lisandro Martínez, who was introduced at halftime for Dalot, blocked former Manchester City star David Silva’s strike, but the ball ricocheted off his leg onto his arm, leading referee Marco Di Bello to give the visitors a controversial penalty.
La Real’s summer signing Méndez made no mistake and set the Spanish club on its way to a first victory on English soil in European competition.
Despite the famous win, the match ended in muted fashion following the day’s earlier news. All post-match interviews were also canceled out of respect for the Royal Family.
The result leaves Ten Hag’s side in third in Group E following Sheriff Tiraspol’s 3-0 win over Omonia Nicosia in the other match in the group.
The Dutchman will have to quickly move past the loss as United prepares for a trip to Moldova to face Sheriff on September 15.