Winning the UEFA Europa League trophy or the German Bundesliga title isn't a yardstick to measure Xabi Alonso's managerial and tactical strength if you ask me because considering what he's already done this season, he's proven beyond every reasonable doubt that he has all the potentials to become a great manager in any top club in Europe. Alonso was able to significantly improve the performance of Bayer Leverkusen in less than three years at the club. Currently Bayer Leverkusen is topping the German league table and if they can continue with their current performance, I think they'll win the league title at the end of the day
Now as long as he manages to lift Leverkusen there should be nothing to worry about as Xabi will be seen as quite a successful coach as the new coach he managed to lift Leverkusen from the mud after previously losing their way and being in a disadvantageous position.
Although it is certain that the championship trophy as a complement to success is definitely expected but as long as he can still play well until now even if he doesn't get a trophy I think it's very good for Xabi.
Now the opportunity is still there for him as a tribute to the club if he manages to lift the trophy as well as to make his name which has shot up and is predicted to be a better coach in the future but even if he still can't realize it I think he will still be very well seen as a coach.