The darkest point is supposedly before dawn. Did Harry Maguire’s late equaliser cast Manchester United’s 3-0 loss against Tottenham as this? Because beforehand, Erik ten Hag was leading his men into an ever bleaker gloom. From 2-0 up to 2-2 in seven first-half minutes, to allowing a third that seemed to kill off United, then to Bruno Fernandes’s second red card in two matches. This seemed to have Ten Hag’s tenure heading for oblivion.

It may still do: the Dutch manager likes to quip he is no Harry Potter. The escape here with a draw has him, for tonight, as Harry Houdini. But for how long can he escape the Sir Jim Ratcliffe axe? It is now only one victory in 10 European outings, so for the next episode of the Ten Hag soap opera tune into United’s trip to Aston Villa on Sunday. Unlike against Spurs, United had the dream start, thanks to Marcus Rashford. Zigzagging in from the left, his package of pace and tricks took him into Porto’s area. The shot was fierce but on Diogo Costa’s near side so he should have repelled it. Instead, the ball squeezed under the goalkeeper’s body and in.

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For the 2,500 journeying fans it was party time. For the rest of the 50,000-capacity crowd, a sinking feeling that punctured the Estádio do Dragão’s party vibe. Swirling smoke, lights, epic-sounding song and glittering silver rectangles held aloft had been the pre-tie show aesthetic. Now United were on the up, and Porto looked to regroup.

Alan Varela swung in a corner from the right and Rashford chest-controlled and hoofed the ball away. Back into United’s area it dropped and Matthijs de Ligt helped cleared. Further robust defending came from Lisandro Martínez: Samu Omorodion received the ball before André Onana’s goal, spun 180 degrees and let fly, but the centre-back was there to block.

This was all feelgood stuff from United, and the sensation heightened. Ten Hag had fielded a leggy-looking midfield in Casemiro and Christian Eriksen, with Kobbie Mainoo unavailable and Manuel Ugarte on the bench. But Eriksen’s early season form has been bright and he produced a muscular burst through the centre circle and a no-look pass that found Rashford. He, too, has been on song and the sweetly effective pass that played in Rasmus Højlund was the latest illustration.

The Danish striker, in a first start this term, unloaded a shot and once more Costa conceded at his near post. Højlund ran to United’s bench to celebrate with David Binningsley, a physio crucial in his recovery from a hamstring problem.

Could this ascendancy last? No, came the answer quickly. United reverted to disintegration mode, allowing Samu to rise and head at Onana: the keeper clawed the ball out but only into Pepê’s path and he stooped to finish. The arena volume powered back up and Porto soon equalised. Again, it was too easy. Close to his area a hapless De Ligt miskicked and the ball spiralled up. His attempted clearing header failed and possession was worked to João Mário on the right. A chip was met by Samu, who headed home; De Ligt, the nearest defender and, again, ineffective.

De Ligt was having a nightmare. A further fluffed kick away handed Porto possession and United barely survived the foray this allowed. Their best hope was attack. Højlund rampaged down the left and marginally missed with an across-goal pass for Amad Diallo. Then, he and Casemiro could not finish at close range when the ball dropped between them. Rashford closed the half with a jet-heeled run but dallied and that was attack over.

A 45 minutes crucial to Ten Hag’s job prospects began with Rashford removed for Alejandro Garnacho, due to “rotation” reasons, and not for his role in allowing the cross that led to Pepê’s opener, the manager explained.

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Still United’s flimsiness remained as, from deep, Francisco Moura raced down the left channel as Micky van de Ven did to create Spurs first goal on Sunday. Moura shot and Onana saved. Worse ensued. This time the raid was along Porto’s right, by Pepê, who Martínez could not catch, and whose cross was rifled in by Samu; De Ligt the patsy once more, unable to get to the striker. United were shellshocked.

Ugarte, bought to stymie attacks, was still not introduced. Ten Hag explained why this was, because he still has to be “integrated” into the “team model”. Garnacho, on his left flank, was United’s best hope of finding an equaliser: a preposterous line to write after being 2-0 up and cruising.

Fernandes was sent off for a second yellow for a high boot – Ten Hag stated this was harsh. Then came Maguire’s intervention via a powerful header from a corner. United: very surreal under Ten Hag. A hugely likeable man, he continues to walk a tightrope.