It doesn’t matter if he wins the Europa League or not. I have never seen such an improvement mid season in my life. We were regularly losing or drawing against the weak sides of La Liga, playing horribly, being totally humillated against strong teams and even not so strong ones like Benfica.
Now we are back dominating and beating teams like Real Madrid, Atlético de Madrid, Sevilla, Napoles, Valencia, Villareal, etc. While also playing great attacking football. We went from literally being 9th in the league, below teams like Osasuna and Rayo Vallecano to being second and trashing Real Madrid. If Xavi had started the season instead of Koeman I have no doubt we would be in the race for the league title.