Yes I agree with you that Xabi Alonso actually lack the big stage experience like the Europa league finally. But I don’t blame him too much because even the greatest managers with fast experience do loose whenever they switch off even small. My first example will be Alex Ferguson beaten by a young Jose Mourinho with Porto and also Guardiola beaten twice in the UCL final. And on both occasions the most experienced was Alex Ferguson. Gurdiola also lost to first timer Tuchel when Chelsea defeated Manchester City in the UCL finally. So sometimes it is not about the coach it is about the players mentality too.
Xabi Alonso set up the team just like he had been doing all season that made them go 52 games unbeaten, so he didn’t do much there, if he has set up the team differently it would have been a gamble. The thing that made Leverkusen to loose the match was that the players lost focus and felt that their were on top of the world. Atlanta were composed and benefited from their composure. If you ask me I will say they were better overall in the competition and deserved it more; Atlanta. They passed the more difficult route to the final
Of course, Xabi doesn't have a lot of experience in the European league, but he has shown that he succeeded in bringing Leverkusen into the final and becoming runners up. Of course, that has proven the best quality he has.
Apart from that, in terms of the changes that Xabi Alonso made to Leverkusen, it was certainly very successful because Leverkusen was previously a fairly weak team. But this season he succeeded in turning Leverkusen into a fairly strong team in the European league and also in the domestic league.
In setting the strategy, of course Xabi has succeeded in doing it, but I think he also needs to have more quality players to help him next season. A team that can be said to be strong, of course, must be able to prove it in the coming season because that is the true level of strength if they are able to maintain the stability of their game. Xabi can of course bring many trophies to Leverkusen provided that he must be fully supported by the management in the transfer market to recruit several players of the best quality.