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.