As the two teams emerged from the tunnel, Anfield was braced for an explosive Bonfire Night encounter.
Having rejected Liverpool's approach to replace Jurgen Klopp, Alonso and his newly crowned German champions had a point to prove against his former club.
So too did Arne Slot, whose record-breaking start was on the line against Europe’s prestigious young coach.
Mediocre opening on Merseyside
Instead of Darwin Nunez, Slot opted for Diaz to lead the line, with Cody Gakpo and Mohamed Salah occupying the flanks.
The Reds dominated possession in the opening stages, with Leverkusen defending tactfully and unafraid to commit men forward on the counterattack.
A highly contested first half meant that most of the action took place in the middle of the park.
Aside from Gakpo’s two wayward efforts and Jeremie Frimpong’s disallowed goal for handball, the first half yielded little to no highlights.
Slot's second-half sucker punch
Much like the Brighton fixture, Anfield came to life after the hour mark to inject some much-needed urgency into the home side.
Almost instantaneously, Curtis Jones’s perfectly weighted through ball bypassed Leverkusen’s defence and found Diaz, who performed an audacious chip over Lukas Hradecky to open the scoring.
The Reds quickly found their rhythm and fluidity in attack, unravelling the otherwise rigid Bayer Leverkusen backline.
In less than five minutes, Salah found himself leading the counterattack and found Gakpo at the back post, who headed in to make it 2-0, only for the offside flag to be raised.
However, the VAR found that Pierro Hincapie was playing the Dutchman onside and the goal stood.
In acres of space, Salah’s in-swinging cross was taken superbly by Diaz, as the Colombian’s deft touch in the box set himself for an easy finish past a hapless Hradecky to take the game out of Bayer Leverkusen’s reach.
Refusing to down tools, it was Alexis Mac Allister on the edge of his box who released substitute Nunez into acres of space, for his effort to be blocked by Edmond Tapsoba but fortunately prodded in by Diaz to complete his hat-trick.
Alonso’s side was left shell-shocked at full-time, condemned to defeat for just the third time in 2024.
The victory propelled Liverpool to the summit of the Champions League table, complimenting their domestic standing.
It was Arne Slot’s fourteenth win in sixteen games in all competitions and a statement performance from the Reds to Europe’s elite, in that they were ready to challenge on multiple fronts.