Yes, their defeat in the Europa League final was a very valuable lesson for Leverkusen, especially Xabi. They can learn a lot from there to be able to make themselves even better when they face more difficult competition next season, namely in the Champoins League. Their defeat in the Europa League final was surprising, because anyone would definitely make Leverkusen the main favorite. But at the same time they had to suffer defeat and that was 3 goals without reply, something I have rarely or never seen during this season.
In the Champions League everything will be much more difficult than what they experienced in the Europa League. They must be ready for all of this, and they can make preparations as quickly as possible. Xabi must ensure that his team is fully ready to face the next season. He also has to ensure that his team's mental condition is in very good condition, because whether they like it or not, the pressure in the Champions League will be much heavier than what they are feeling now.