Cody Gakpo was also on target as the Reds maintained their 100 percent record in Europe after four games.

Alonso, a Champions League winner during his playing days in the Liverpool midfield, turned his back on succeeding Jurgen Klopp to remain at Leverkusen after leading them to an unbeaten German league and cup double last season.

Liverpool, though, are not left wondering what might have been as Arne Slot continued his stunning start as manager with a 14th win in 16 games in all competitions.

Leverkusen remain on seven points from their opening four Champions League matches after another sobering night for their ambitions to match the heights they hit last season.

Alonso's men sit fourth in the Bundesliga, seven points adrift of leaders Bayern Munich, and failed to master what Alonso described pre-match as a "beautiful challenge".

The Spaniard cut a frustrated figure on the touchline as his side struggled to get into the slick passing rhythm they have become famed for during his tenure.

The German champions, though, created the best of what chances there were in a first half short on goalmouth action.

Jeremie Frimpong saw appeals for a penalty waved away when he went down with a clear sight of goal under a challenge from Kostas Tsimikas.

Frimpong also had the ball in the net just before the break but handled before racing through to finish.

Liverpool's best opening of the half came seconds later when Curtis Jones' excellent pass finally found some space for Gakpo, but his powerful effort was repelled at his near post by Lukas Hradecky.

Just like in Saturday's 2-1 win over Brighton that took Slot's men to the top of the Premier League, it took until the second half for both Liverpool and the Anfield crowd to come to life.

Mohamed Salah should have done better when he sliced wide after a jinking run through the Leverkusen midfield by Ryan Gravenberch.

A cagey tactical battle was sparked into life on the hour mark by a moment of stunning quality.

Jones spun his marker before splitting the Leverkusen defence with a pinpoint pass for Diaz, who nonchalantly chipped the advancing Hradecky.

Moments later, Liverpool had killed the visitors off with another flowing move.

Salah's driven cross was headed in at the back post by the flying Gakpo.

The linesman raised his flag to momentarily silence the celebrations, but a VAR review showed the Dutch forward was onside for his sixth goal of the season.

Victor Boniface headed wide with a huge chance to bring Leverkusen immediately back into the game.

Instead, Liverpool pulled further clear seven minutes from time when Diaz controlled Salah's looping cross and fired home.

The Colombian then rounded off the scoring with his ninth of the season in stoppage time with another clinical finish after Darwin Nunez's blocked shot fell into his path.

Liverpool's return of 12 points means they are almost certainly already assured of a place in the knockout phase, but are closing in on sealing direct progression to the last 16 via a top-eight finish in the 36-team league.