A horrible error from Benfica's young record-breaking goalkeeper Mile Svilar gifted Manchester United a 1-0 win at the Estadio da Luz as Jose Mourinho's men made it three victories from three in Group A.
Svilar made history as the Champions League's youngest-ever goalkeeper at 18 years and 52 days, but his disappointing positional judgement resulted in Marcus Rashford scoring the decisive goal 26 minutes from time, letting United off after a pretty poor performance.
United probably just about edged things before the break, but it was a turgid half of football, with neither team showing a great deal of quality in the final third.
Mourinho will have been particularly unimpressed with United's inability to test Svilar regularly, particularly given how uncomfortable he had looked with set-piece deliveries.
United were certainly more forward-thinking in the second half and did create a few chances, but they were still nowhere close to their best, with Romelu Lukaku starved of effective service.
But luckily for the Premier League side, Svilar accidentally carried a Rashford free-kick over his line in the 64th minute and that proved to be enough for the visitors, as they strolled to victory and left pointless Benfica, who had Luisao sent off in injury time, needing a significant turnaround to reach the knockout phase.
United were guilty of a thoroughly underwhelming start, as they offered nothing from an attacking perspective and invited Benfica pressure on to themselves.
The hosts almost took advantage of United's setup in the 14th minute, but Eduardo Salvio smashed a left-footed effort considerably wide from 12 yards after Alex Grimaldo had brilliantly dribbled past three challenges, linked with Pizzi and crossed into the centre.
That scare did not exactly jolt United into life, as it took them another 17 minutes to even craft a minor opportunity, with Rashford heading Daley Blind's cross well over.
United finally tested the inexperienced Svilar for the first time nine minutes before the interval – Nemanja Matic drilling an effort at him from a narrow angle after driving into the left side of the area.
A poor Douglas pass then gifted United the chance to break just before half-time, but Henrikh Mkhitaryan sliced his eventual shot well wide from the edge of the area.
United looked far more urgent at the start of the second half and Rashford forced Svilar into a save with a fierce 25-yard volley in the 50th minute.
But they continued to struggle to bring Lukaku into the game and Matic's 30-yard strike – which went just over – seemed to highlight United's frustration.
Rashford then almost scored directly from a corner after an hour, with Svilar punching it out from under the crossbar just in time after initially looking well out of position.
Svilar's poor judgement was ultimately punished a few moments later, however.
Rashford curled a free kick in from the left flank and it appeared to sail harmlessly through to Svilar, but he stepped over the line with his ball in his grasp, leaving coach Rui Vitoria shaking his head in dismay.
The England international was then forced off with a seemingly innocuous injury with 14 minutes to go, but it had little impact on the final result.
United successfully held on to what proved to be a straightforward win, with Luisao's red card for second bookable offence after a rash tackle on Scott McTominay compounding a bad day for the home side.