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 among the football fans who have high expectation on a certain club. At Leverkusen, Xabi Alonso the most popular name, unlike at other big clubs that have big star of players which can become a scapegoat when their team underachieve something expected. Most football fans other than Leverkusen fans don't even know Leverkusen players.
The team being under pressure is something not being understood by average football fans. They only see what and how the team perform on the pitch, and blame the known players when they make mistakes or blame the coach when the team underperform and lost. I observe this kind of behavior is attached to the fans who put big bets on the lost team.