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The European heavyweights are at very different stages right now, with the perennial Bundesliga champions second favourites to win a competition that the Red Devils did not even feature in last season.

Absentee-hit United began brightly enough at the Allianz Arena, only for Serge Gnabry to score shortly after ex-Manchester City forward Leroy Sane’s effort squirmed past summer signing Onana.

Rasmus Hojlund netted his first goal for the club but Kane quickly slammed home a penalty, with Casemiro scoring either side of Mathys Tel’s stoppage-time strike as the Red Devils suffered a fourth defeat in five matches.

Few could argue with the eventual outcome, but United had started brightly in Bavaria and should have gone ahead through Facundo Pellistri or Christian Eriksen inside four minutes.

Bayern, who were without suspended boss Thomas Tuchel on the touchline, shook off initial sluggishness, with Sane’s strike beating distraught Onana before Gnabry fired home four minutes later.

Hojlund’s effort off the heel of Kim Min-jae shortly after half-time brought hope to Erik ten Hag’s men, only for Eriksen to soon be adjudged to have handled.

Ex-Tottenham team-mate Kane scored the resulting spot-kick, with Casemiro’s brace sandwiching Tel’s strike during a dramatic conclusion to the Group A opener.

Bayern had won their Champions League opener in 19 successive seasons heading into this match in Munich, where they had not lost a group-stage game since 2013.

Few expected those impressive runs to end on Wednesday, although United began purposefully and should have taken an early lead.

Eriksen collected a lax Sane pass just outside the edge of the box and sent a smart, slightly deflected ball to the far post, which was cut out just in the nick of time.

Alphonso Davies’ challenge prevented Pellistri scoring on just his second United start, with Eriksen firing the follow-up straight at Sven Ulreich.

Ten Hag’s team continued with confidence in and out of possession as they looked to keep things tight while offering an attacking threat.

Bayern were looking uncharacteristically sloppy in key moments, only for an Onana clanger to gift them a 28th-minute lead.

Sane cut in from the right and collected a return pass from Kane, before hitting a first-time snapshot that too easily beat the United goalkeeper.

Devastated Onana buried his head in the turf and team-mates came over to console him – a gut punch followed by a body blow four minutes later.

Impressive Jamal Musiala drove down the left and kept his cool under pressure in the box, laying back for Gnabry to sweep home.

Sane went close to a third at the other end before the break, which United returned from strongly as they scored in the 49th minute.

Quick play after gaining possession ended with Hojlund turning and hitting an effort that went in off Kim to the delight of the United fans high at the opposite end of the ground.

But Bayern wasted little time restoring their two-goal cushion. Musiala saw a shot deflected just wide and play was then halted as the VAR checked for a potential penalty.

Referee Glenn Nyberg was advised to review an Eriksen handball from Dayot Upamecano’s header, with the official pointing to the spot after watching the incident back.

Kane stepped up and smashed home the resulting spot-kick into the bottom left-hand corner beyond the reach of Onana.

Sane saw an effort come back off the post as Bayern sought an immediate fourth as United tried to batten down the hatches.

Good work by Musiala ended with Onana stopping a Kane thumper, with Lisandro Martinez booked for barging over the striker in the aftermath.

United’s goalkeeper stopped a Sane 20-yarder dipping in as the visitors hung on, with Onana pushing away a goalbound Eric Maxim Choupo-Moting attempt.

He and fellow substitute Kingsley Coman had further chances, before United replacement Alejandro Garnacho turned wide at the other end.

The visitors found the net in the 88th minute as Casemiro managed to direct home having failed with his initial effort, sparking a box-office conclusion.

Thomas Muller hit the post in stoppage time, before fellow introduction Tel fired into the roof of the net.

Casemiro headed in a Bruno Fernandes free-kick but there was no time to snatch a draw.