Cody Gakpo found the back of the net as the Reds continued their flawless record in Europe with a perfect four wins out of four.
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 the Nov 2 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.