The high expectation on Leverkusen made people putting blame on Alonso for the loss. I think it's an expression of disappointment that makes people find anyone to put the blame on. I think it's indeed unfair to blame Alonso when he was under his own pressure to stay in unbeaten record that forces himself pushing the limit on his team which cause mentally fatigue in the Europa League final.
It would be inappropriate for anyone to blame a coach like Xabi Alonso who has looked after and brought the Leverkusen team to an even more extraordinary level this season. Because Leverkusen has also succeeded in winning two trophies this season even though they lost at the wrong time in the final match of the Europa League this season. And I think people who are willing to blame Xabi Alonso just because Leverkusen lost the final match are people who don't understand the pressure that Xabi Alonso is under and maybe also don't understand how football works for all teams including Leverkusen.
It has become a common thing that when there is failure, previous successes are forgotten and those who blame Xabi Alonso are those who forget that it is impossible for a team to always win and vice versa, namely continuing to lose. The defeat was indeed surprising for many people and also quite disappointing including for bettors who chose them to win and it seems like Bayer Leverkusen did not expect that Atalanta could play so well and of course it was beyond their expectations that is why Bayer Leverkusen failed to penetrate Atalanta's defense even though they could control the game but Atalanta played very neatly and when they had the opportunity to attack their attack was effective in scoring goals and that could not be achieved by Bayer who had to accept their first defeat after a smooth journey to become the previously unbeaten team.