Don't believe the scoreline – this was not one of Manchester United's brightest European displays, even before a power cut plunged the stadium into darkness and forced the players off the pitch for 10 minutes in the second half.

United were a goal down at that point thanks to Alan's penalty, and until going behind had never managed to cause any serious problems for the Braga defence. The intention was clear – the visitors had no real need to win the game and could afford to sit back, try to contain and see what the Portuguese team could do – but Braga had shown themselves capable of penetrating United's defence at Old Trafford and proceeded to do so again.

Only when United brought Robin van Persie on and reverted to something like their usual set-up did the goals to win the match arrive in the final 10 minutes. It was a lesson to Sir Alex Ferguson, if another one were needed, that his side usually do better when instructed to attack.

When Javier Hernández added a final goal in injury time, his third in two games against these opponents, the final score made a mockery of what had gone before. It was almost robbery. For more than 70 minutes Braga had been the only side playing any football, yet United's late show not only ensured a passage to the knockout stage but confirmed them as group winners.

"Winning by that score was a little bit surprising," admitted Mike Phelan, appearing in place of the manager, who was on an earlier flight home. "All we did in the first half was keep possession. We only got started when we upped the tempo, and Robin van Persie was instrumental in getting us further up the field."

Surprisingly, in view of the fact that Ferguson blamed a lack of width for his team conceding two early goals in the home fixture, United went narrow again against Braga, with only Nani used as a winger. Ryan Giggs was retired from touchline duty some years ago and operated behind Wayne Rooney in central midfield, while Antonio Valencia found himself pressed into service at right-back. It appeared from the Uefa teamsheet that the diamond formation was to return, yet that was a misleading impression. United were simply flat, almost unrecognisably so, as if they were attempting to bore Braga into submission.

The home side gave United a warning by hitting a post midway through the first half, once again showing their ability to cross quickly and accurately. Hugo Viana put in a low cross from the left, Eder met it perfectly with a diving header that David de Gea could not reach, and the goalkeeper must have been relieved to see the ball come back into play off his right-hand upright.

Although Braga did not create anything quite as clear-cut in the minutes before half-time, they continued to look the side more likely to open the scoring, mainly because United were playing so conservatively. Two stats that flashed on the scoreboard just before the interval told the story. In the first period the home team had enjoyed only 40% of possession but had managed 10 goal attempts to United's one.

The character of the game changed when Braga took the lead straight after the interval. Jonny Evans did little more than stand his ground to sent Custódio flying, though United were already playing a dangerous game by letting their opponents pass their way into the area and the referee showed little hesitation, allowing Alan to blast his side into the lead from the spot.

The question now was whether Ferguson would continue to play a patient game or send on the missing goal threat in the shape of Van Persie. It was a question that was not immediately answered, because shortly after Danny Welbeck had wasted some excellent right-wing work by Valencia by controlling the cross with a hand, the lights went out with 57 minutes played. Seven minutes after the restart following the blackout, the Dutchman appeared for the ineffective Welbeck and the effect was almost instant.

With Rio Ferdinand on for Evans and Rafael da Silva's introduction allowing Valencia to move back to the right wing, United were much more like their normal selves, and when Giggs spotted the Braga goalkeeper off his line 10 minutes from the end and found Van Persie, the equaliser was soon delivered, quickly followed by the visitors' second goal when Rooney was tripped in the box as he went for the rebound after Hernández saw a shot blocked. Rooney took the penalty himself, after missing one at the weekend, and calmly ensured his side's progression. "We were surprised he took it," Phelan said. "But he took it very well."