When subs go wrong
Erik ten Hag has earned a lot of praise for his substitutions this season, but the triple change he made after 62 minutes of this game will not fall into that category.
United had started the second half a little complacently, but the changes robbed them of quality as well as a major goal threat and allowed the La Liga side to continue growing in confidence.
Anthony Martial might not have had more than an hour in his legs, but taking Bruno Fernandes and Jadon Sancho off changed the dynamic in United's front three and might also have sent a message to Sevilla, that United were looking to hold what they have.
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Anthony Elanga and Wout Weghorst came in the front three and it made United easier to defend against. Neither are much of a threat in front of goal. It was a risk replacing that trio of players in one go, but it was an even bigger gamble when the drop off in quality is so stark, with the notable exception of Christian Eriksen.
The score was 2-0 at that time and United still looked dangerous going forward. A third goal would surely have buried the tie ahead of next week's return leg, but instead, those changes changed the course of this game in Sevilla's favour.
There was a stroke of luck about how the ball ended up in the United net for both goals, but this was a collapse you felt was bought on by those changes and the way it turned the tide of the game. There was misfortune for United, but so much of it was self-inflicted.
Defensive disasters
As if the way United collapsed and turned a routine second leg into a daunting trip to Seville wasn't bad enough, they now face the prospect of losing two more key players for a vital part of the season.
The sight of Marcus Rashford sat in the stands was bad enough - and there's an argument that failing to score more than two when they were on top was partly caused by his absence - the loss of Raphael Varane and Lisandro Martinez to injury is nothing short of disastrous.
Varane went down in the first half holding his leg and was replaced by Harry Maguire at half-time. The World Cup winner is prone to injury problems and for all his quality he can't be relied upon to stay fit for an entire season.
The injury to Martinez was more unfortunate and it looked more serious as well. He was helped off the pitch by his World Cup winning teammates Gonzalo Montiel and Marcos Acuna but was then carried down the tunnel on a stretcher. His absence also meant United were down to 10 men.
Ten Hag will have to wait for a clearer assessment on the extent of those injuries, but it could mean a return to the partnership of Harry Maguire and Victor Lindelof, with nothing in reserve. That comes at a time when the games are coming thick and fast, with next week's game in Seville now looking difficult and an FA Cup semi-final to follow three days later.
Midfield job swap
Before it all went so bewilderingly wrong there had been a fitting symmetry to United's opener and it was a goal that would have delighted Ten Hag and his coaching staff, who have persisted with the experiment of swapping Fernandes and Marcel Sabitzer and are now beginning to see rewards.
After something of a false start against Brentford, it worked well against Everton but their combination for the opener on Thursday was real vindication that they can make a success of this approach for now.
Fernandes has taken to his deeper role and he was creative from that position against the Toffees, usually with sweeping balls from deep. He showed his vision when threading a pass to Sabitzer against Sevilla. The Austrian was playing on the last line of defence and turned sharply before getting his shot away, finding the back of the net via a deflection.
It was notable how many advanced attacking positions Sabitzer was taking up. His second goal came as he surged past every other United forward. With Martial holding the ball up, he ran into the centre-forward position and timed his run to perfection. The finish wasn't bad either.
This was Sabitzer's fifth successive start for United, having only started four games for the club until this run. There is something to be said for the idea that he has now found his rhythm, having not played much for Bayern Munich before his loan move to Old Trafford in January. United are now seeing the best of him.
There is a decision to make in midfield for Ten Hag, who has Christian Eriksen on his way back to full fitness. He's probably not ready to start just yet, but it's only a matter of time. When he is at that stage he might find Sabitzer hard to dislodge on current form.
Antony's form
It's tempting to say this was a night when we saw the best and the worst of Antony, but such is the winger's style that even his very best days are likely to include some moments that leave you scratching your head.
This was certainly a more consistent display and in recent weeks he has begun to look like the threat United want him to be on the right. There is a little more variety to his game and he was much more confident when looking to take players on against Sevilla.
Perhaps that confidence manifested itself in the wrong way midway through the second half, when he tried his spin trick in a pointless area and simply lost possession, before getting involved in a confrontation straight after it with Sevilla's Argentine left-back Marcos Acuna. Their battle was one that ran for most of the second half.
He clearly needs to become better at knowing when the time is right to get the skills out, but this was a better all-round display. He took one touch too many to release Jadon Sancho in the first 30 seconds, but continued to draw defenders towards him and deserved a goal when his second-half shot cannoned back off the crossbar.
Telles
It was probably no great surprise that Alex Telles was only on the bench for Sevilla on his return to Old Trafford, but it does suggest that United might be lumbered with the Brazilian once again this summer.
He's started 17 times for Sevilla this season, but Marcos Acuna has tended to be the first-choice left-back. He scored at the weekend but was also sent-off, so Telles can at least expect to start against Valencia at the weekend.
The 30-year-old doesn't have a future at Old Trafford, given Luke Shaw has just signed a new deal, but he might well point to the troubles that his replacement, Tyrell Malacia, is having and suggest he could do a job. Malacia was at fault for the goal that has given Sevilla a lifeline in this tie.
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