Manchester City suffered a stunning 2-0 defeat at the Etihad Stadium on Tuesday night, turning what should have been a celebratory occasion into a nightmare.
The match marked Pep Guardiola's century of Champions League fixtures at the helm of the Citizens, yet it concluded in bitter disappointment.
Bayer Leverkusen capitalised on City's lacklustre display to secure an impressive away victory.
Alejandro Grimaldo broke the deadlock with a clinical strike before Patrik Schick sealed the German side's triumph with a well-taken header.
Manchester City were toothless in attack as Bayer Leverkusen took advantage
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The result leaves the English champions reeling from consecutive defeats following their weekend setback at Newcastle United.
Guardiola's radical team selection proved costly as he reshuffled his entire outfield following Saturday's Premier League setback. The wholesale alterations left City's performance disjointed and lacking fluency.
The hosts dominated possession early on but moved the ball with frustrating lethargy.
Nathan Ake came close when his stabbed attempt forced Mark Flekken into an alert save, yet City struggled to create meaningful opportunities thereafter.
Leverkusen punished their sluggishness through a swift counter-attack. Christian Fofane found his captain Grimaldo, who struck a precise first-time shot past the City goalkeeper to give the Bundesliga side a deserved advantage.
Schick extended the visitors' advantage nine minutes into the second period, rising above City's defence to direct Ibrahim Maza's accurate delivery past the goalkeeper.
Alex Grimaldo scored Bayer Leverkusen's first on a shoddy night for City
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The Czech forward's header left the home side facing an uphill battle.
Guardiola responded by introducing Erling Haaland after an hour, alongside Phil Foden and Rayan Cherki in desperate attempts to salvage the match.
The Norwegian striker nearly made an immediate impact when Foden played him through, only for Flekken to smother the opportunity.
Man City have now lost two in a row
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City's peripheral players disappointed throughout. Omar Marmoush, Oscar Bobb and Savinho squandered their rare starting opportunities, producing ineffective performances when given the chance to impress in attack.
The defeat carries serious ramifications for City's European campaign. The result sees them tumble to sixth position in the league phase standings, with their place in the top eight now under threat.
Should Wednesday's matches produce unfavourable outcomes, Guardiola's side could find themselves outside the automatic qualification spots.
This would force them into additional playoff fixtures to secure knockout stage progression.
Erling Haaland isn't happy as he watches on here...
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Patrik Schick guides his header into the bottom corner to double Bayer Leverkusen's lead against Manchester City 😲 pic.twitter.com/4xRwfx58MG
The loss also terminated City's remarkable sequence of thirteen successive home triumphs over German opposition in Europe's premier competition.
Kasper Hjulmand's Leverkusen departed Manchester with a historic victory, their jubilant supporters celebrating a memorable night at one of English football's most formidable venues.