Manchester United's winning run was broken as a Brais Mendez penalty saw Real Sociedad earn a 1-0 victory in their Europa League opener at Old Trafford on Thursday night.
A tight game was decided when substitute Lisandro Martinez was penalised for handball in the area despite the ball hitting his knee first, allowing Mendez to convert from the spot.
Sociedad have previously endured a poor record against English teams in Europe, failing to win any of their six matches, but held their hosts to a goalless first half.
The United side included Casemiro - making his first start for the club - and recalled duo Harry Maguire and Cristiano Ronaldo. The Portuguese forward had a few half-sights of goal and saw an excellent header disallowed for offside.
Anthony Elanga enjoyed a few electric runs out wide, but Erik ten Haag's side recorded just one shot on target as Sociedad frustrated their hosts going into half time.
Ronaldo squandered United's first chance of the second half, not getting enough on substitute Bruno Fernandes' pinpoint cross from the right.
The tight battle swung the way of the visitors shortly before the hour mark in somewhat fortuitous circumstances. A shot from David Silva was blocked by diving substitute Martinez before the ball ricocheted onto his arm, with referee Marco Di Bello pointing to the spot.
Mendez made no mistake, lashing his effort beyond the reach of David de Gea.
The Spaniard was called into action soon after as he batted away Takefusa Kubo's drive before Casemiro spurned a decent opportunity, heading the ball into the ground and over the crossbar at the back post.
United pushed on and the roaming dribbling of Martinez pushed them into promising positions, but it was Sociedad who nearly doubled their lead instead, though Alexander Sorloth butchered a counter attack and allowed De Gea to collect the pass.
It remained 1-0 and ended United's recent winning run in the Premier League which stood at four victories.
Manchester United player ratings vs Real Sociedad - Europa League
1. Man Utd (4-3-3)
GK: David de Gea - 5/10 - No saves to make and only four touches in the first half. More active in the second period but couldn't save the penalty.
RB: Diogo Dalot - 7/10 - Used the ball well in the final third, whipping in some dangerous crosses.
CB: Harry Maguire - 6/10 - Back in the starting XI and thankfully for his sake not a difficult first 45.
CB: Victor Lindelof - 6/10 - A little tentative but used the ball accurately and made a couple of interceptions.
LB: Tyrell Malacia - 7/10 - Another forthright and front-foot defensive performance.
CM: Fred - 5/10 - Arrived in good positions from midfield but a couple of sloppy giveaways relinquished control.
CM: Casemiro - 6/10 - Wasted a good opportunity to equalise with a mistimed header. Gloriously spun by Aihen Munoz in a tough moment.
CM: Christian Eriksen - 7/10 - Quality of delivery was brilliant. United's play now firmly runs through the Dane and they're all the better for it, despite Thursday's result.
RW: Antony - 6/10 - Tried to link up with his attacking teammates but was rarely successful.
ST: Cristiano Ronaldo - 5/10 - Had an excellent header disallowed for offside. Missed another header he should have buried at the start of the second half. Didn't look like he enjoyed playing in United's Thursday night team.
LW: Anthony Elanga - 6/10 - A couple of brilliant runs made him United's main threat in the first half. Threat waned after the break.
SUB: Bruno Fernandes (45' for Eriksen) - 6/10 - Immediately set Ronaldo up with a cross. Increased United's tempo for a period.
SUB: Lisandro Martinez (45' for Dalot) - 6/10 - Somewhat unfortunate to concede the penalty. Broke forward with purpose late on.
SUB: Jadon Sancho (71' for Antony) - 6/10
SUB: Alejandro Garnacho (71' for Elanga) - 6/10
SUB: Charlie McNeill (83' for Malacia) - N/A
Manager: Erik ten Haag - 5/10 - Switched up his starting XI for Europe. Lack of attacking quality will be concerning for Ten Hag, though he will certainly have a good idea of the players he can trust after Thursday night.