A 2-2 draw in Germany saw Sevilla lose agonisingly 3-2 on aggregate.
Castro declared afterwards: "I am very proud of my players.
"Now we must try to achieve the goal of returning to the Champions League because it is very important for the club.
"The season is good. The team put it all together today, with this attitude we were very close to achieving the goal. Football is football and we also made mistakes. It is the team we want, the one that has made us enjoy. Playing like this I am not worried about the future. Derbi? More than Betis I worry about thirteen games."