It had been difficult to locate any pre-match optimism around Manchester United. The club has sunk so low, so quickly in the early weeks of the season that it was almost as if their supporters would accept a defeat without humiliation on their Champions League return.

It was what they got. There were flickers from United in the opening quarter of the tie and the tonic of a first goal from the summer signing, Rasmus Højlund, to make it 2-1 at the beginning of the second half. At least United fought until the end, something that the beleaguered Erik ten Hag will cling to in the days ahead.

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Casemiro offered the hope that United could snatch a draw, scoring on 88 minutes for 3-2 and jolting Bayern Munich, who had been toying with their opponents. Casemiro would score again with virtually the last action – after the Bayern substitute Mathys Tel had made it 4-2. The goals were big for Casemiro, who has laboured so far this season, the second a flicked header from Bruno Fernandes’s free-kick.

Bayern know a thing or two about United’s ability to come back from the brink but there was barely time for a final kick-off. Moreover, this is a different era, a different United team and the greater takeaways from their performance were individual errors and an overall softness.

United only seem to play in fits and starts at the moment. Their self-belief is at an awfully low ebb – it was as if they, too, expected a fourth loss in five games – and their numerous injured players cannot return quickly enough.

The image of the evening was that of the goalkeeper, André Onana, with his face buried in the turf, burning with embarrassment after spilling a tame Leroy Sané shot into his own goal for 1-0.

The inevitable Harry Kane goal celebration was not far behind. Kane got the assist for Sané and, when Christian Eriksen was penalised for handball shortly after Højlund’s goal, the England captain buried the penalty. Kane has five goals in five starts for his new club.

Serge Gnabry had scored Bayern’s second and they had the chances to add to their lead before Casemiro slipped and still managed to steer home for his first. It was a bizarre ending because at 3-2, Thomas Müller – on as a substitute – hit the post on a Bayern counter before Tel further burnished his fast-rising reputation with a cool finish.

The reality was that United’s away-day woes in the big games under Ten Hag never looked like stopping after Onana’s aberration; Bayern never looked like suffering a dent to their imposing group stage record here.The perennial German champions were unbeaten in their previous 27 home Champions League group ties and they had won their opening fixture in each of the past 19 campaigns by an aggregate score of 47-2.

Ten Hag had been bold with his approach at the outset, Fernandes and Eriksen pushed up in central midfield, Facundo Pellistri given his chance on the right wing. The 21-year-old almost enjoyed the dream start only to feel Alphonso Davies make a saving tackle on him. When the ball broke, Eriksen drilled low for goal. Sven Ulreich saved smartly.

Ten Hag’s first lament was that his team did not get anything from their early promise, which was marked by some slick passing. Instead, Bayern stirred, with Lisandro Martínez required to race back to win a one-on-one tackle with Gnabry.

The breakthrough goal was a body blow and if Onana’s howler will take the headlines, the defending on the United left was so tame. Sané was shown inside by Sergio Reguilón and Marcus Rashford but Eriksen did not make the tackle. Sané was allowed to swap passes with Kane and unload his shot and that was when Onana wanted the ground to swallow him up.

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Bayern were merciless in how they turned the screw, Jamal Musiala running up the inside left and turning into Diogo Dalot, who lost the duel. Again, it was weak. When the ball broke and with Victor Lindelöf sucked across, Musiala pulled back for Gnabry whose finish was true.

Reguilón combined with Casemiro to cross low on 34 minutes; there was nobody there for United. But it was Reguilón’s work at the other end that was the worry. He backed off before the interval and allowed Sané to curl for the far corner. It was only just wide.

It was hard to ignore Ten Hag’s selection crisis. Missing 12 players for various reasons, he named three goalkeepers on the substitutes’ bench and only one defender – Jonny Evans.

United were determined to salvage something in the second half, even if it was pride, and their hopes surged when Højlund took a pass from Rashford and shot quickly, the effort flicking in off Kim Min-jae.

The goal only seemed to make United more vulnerable, Musiala streaking clear straight away to win a corner. From it, Bayern won the penalty and it was harsh, Dayot Upamecano thudding a header at Eriksen’s arm from point-blank range.

Kane did the rest and it was easy to fear the rout thereafter as Bayern created half a dozen good chances, Sané hitting the post with the best one, Onana making some decent saves. The finale was strange and illusory.