Of course this is a huge loss for Leverkusen, because they have to re-arrange their strategy with a new coach. With Xabi, it can be said that they have found their form, proven by still being able to compete at the top even though they could not defend the title. However, even though they could not defend the title, I think it was an extraordinary achievement and it would be very difficult for them to find a coach who could replace Xabi.
I agree with you, things are very different now, now ETH is with Leverkusen and I'm sure this coach will do well, he will forget all the experience he had at MU, since that was his worst performance there, but now with Leverkusen I'm sure he will improve, and if he doesn't the Germans will kick him out, Germany is not England and even less will Leverkusen let themselves go down after having some great seasons.