LIVERPOOL, England -- Xabi Alonso said he "didn't have time for tourism" ahead of his return to Liverpool as Bayer Leverkusen coach, but if he was planning for a happy homecoming, a 4-0 defeat and a humbling at the hands of the man who claimed the job that many wanted him to take was probably the last thing he expected.

"Painful," was how Alonso described his evening. That was something of an understatement. The Liverpool fans sang Alonso's name at the end of the game, but not before they serenaded Arne Slot, the coach who has made a stunning start to life as manager at the club as Jurgen Klopp's successor.

Having secured his status as a Liverpool legend by helping Rafael Benitez's team to UEFA Champions League success in 2005, Alonso was the overwhelming fans' choice to replace Klopp when it was announced that the German would be leaving at the end of last season.

Alonso's decision to commit his future to Leverkusen, who were on course for their first-ever Bundesliga title, was initially met with disappointment by the Liverpool fans. However, the club already had their sights set on Slot, then the Feyenoord coach, and it was the 46-year-old who was chosen to fill the void left by Klopp.

Six months on, Liverpool are top of the Premier League and Champions League, and Slot is the new darling of the Anfield crowd. Nobody is talking about Alonso anymore and after this emphatic win, the debate is officially over.

Slot has made his mark and his Liverpool are threatening to rule in England and Europe this season. Alonso, on the other hand, has struggled to sustain Leverkusen's incredible form from last season, when they won the Bundesliga without suffering a defeat. It is now two wins from eight games and the magic touch he displayed not so long ago has now disappeared. He's still a rising star in the coaching world, but Alonso was taken to school by Slot at Anfield and it was a brutal lesson for the 42-year-old.

A second-half Luis Díaz hat trick and a Cody Gakpo header sealed Liverpool's victory after a cagey first half had ended goalless. But just as they did against Brighton and Hove Albion in the Premier League at the weekend, Slot's team went up several gears after the interval to blow their opponents away.

Díaz was central to that, scoring his first hat trick for Liverpool while playing in an unfamiliar centre-forward role. The Colombia international's first -- a cool chip over goalkeeper Lukas Hradecky following a sublime Curtis Jones pass -- was the pick of his hat trick.

When you analyse the changes that Slot has made since replacing Klopp, they have been subtle rather than significant. The players are the same and not much happens without Mohamed Salah influencing it up front or Virgil van Dijk influencing it in defence. But is it the same as it ever was? Not quite.

Slot's Liverpool are more solid at the back than Klopp's team, with fewer attacking runs by their full-backs, and against Leverkusen, midfielder Ryan Gravenberch sometimes played so deep that he occupied the role of a third centre-half. He was also given the task of shadowing Leverkusen playmaker Florian Wirtz to nullify the creative hub of Alonso's team.

Speaking to reporters after the game, Slot said he had been surprised by Alonso's formation and decision to play striker Victor Boniface in a wide position.

"Leverkusen played without a No. 9 and I have never seen Boniface play from the left," he said when he admitted that it took until the second half for his players to click.

"In the second half, we adjusted the game plan and took more risks."

And that seems to be the key with Slot's Liverpool. They play one way in the first-half before upping the ante in the second half. Klopp's Liverpool were in top gear for 90 minutes and it was brought success and excitement to Anfield, but Slot is finding another way and it was good enough to dismantle the champions of Germany.

"The result is painful, the performance is more painful," Alonso said. "It can happen that you come to Anfield and lose. To lose 4-0 is not nice.

"They are a top, top team and they punished us. It was a pity we couldn't hold 1-0 for 15 to 20 minutes. They did it against Brighton, they did it against us.

"To be a top, top team, we need to go through those difficult moments -- to suffer in moments that are not that nice, especially in the Champions League. That can happen in this competition."

Leverkusen and Alonso are no Champions League novices, but they looked like it in the second half. And that is perhaps the biggest tribute to Slot and Liverpool. They made a formidable team look ordinary.

When Real Madrid visit Anfield on matchday five later this month, they would do well take heed of the warning.