Erik ten Hag chose to bench Marcus Rashford for the clash due to a slight injury worry with Harry Maguire dropped for the returning Victor Lindelof. There were also starts for young Garnacho and Donny van de Beek - his first for the club since December.
Garnacho, an 18-year-old winger previously included in Argentina’s senior squad ahead of the World Cup, immediately repaid his manager’s faith with the opener in the 17th minute.
A fine counter-attack was sparked by Bruno Fernandes’ flicked header towards Cristiano Ronaldo. Robin Le Normand was guilty of standing off the veteran as he laid in Garnacho, who had only one thing on his mind as he belted into the box and buried his first goal for the club in the top corner.
Needing a two-goal win to progress as group winners, United remained somewhat cautious and played largely on the break to ensure Sociedad could not nick a goal, as they did at Old Trafford in September.
Garnacho, however, continued to impress and could have had a second but for a slip as he shot from a similar position.
Ronaldo, albeit possibly offside had VAR been called upon, sent a guilt-edged lob of the goalkeeper onto the roof of the net as the visitors pushed for a second late in the half.
David De Gea barely had a save to make before parrying Andoni Gorosabel’s volley on 43 minutes, Pablo Marin rushing in to net a shoe-in leveller that the goalkeeper somehow turned away his rebound.
Sociedad continued to threaten in the second half, Lisandro Martinez competing well with the towering Alexander Sorloth to divert a header over the bar.
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Ten Hag turned to Rashford for Van de Beek before the hour mark while also making the curious decision to send Scott McTominay on for Lindelof.
Nonetheless, the game began to drift away from United despite a three-man backline set up to chase the much-needed second goal.
Carlos Fernandez sent two half chances over the bar before taking a yellow card for time wasting, referee Georgi Kabakov losing control of the game having handed out a harsh booking to a visibly frustrated Ronaldo.
Maguire was the next to come on, deployed as a striker for the final ten minutes. Plenty of balls were loaded into the box but La Real defended resolutely and successfully ran down the clock to secure first place, sending United into a play-off against the likes of Barcelona.