With Alonso as Real Madrid's headcoach, and a former player of Madrid, and a club which he played for many years, he's going to have a great achievement and success with Madrid, considering his successful managerial record with Leverkusen.
I believe they have higher opportunity in this competition to lift this very trophy aside, PSG and Chelsea two current champions.
Real Madrid is very lucky to have a coach like Xavi Alonso come into their rescue, because they had a poor last season under Carlo Ancelotti, and I do believe Alonso will get Madrid right back on track with his tactical system of play.
First off being a club legend and a good contributor in his time at Real Madrid doesn’t guarantee he will achieve great things or be successful as a manager with the same club it just has to be based on his managerial performance and capabilities. For the sake of his success with Leverkusen which raises the bar for him, then I can say their is a huge possibility that he will be successful but not because he is a legend with Madrid.