I think they made a mistake for taking him as the new head coach of the club and they didn't waste any time in firing him also I like as they realized their mistake on time because if they had chose to let him manage the club for maybe half a season trust me it'd be worse than this, because Erik Ten Hang was actually going to take that team back to an average team in that league something Xabi Alonso took lots of time to build, and my advice for Erik Ten Hang is he should just take a shot break in coaching job or maybe focus on becoming the Dutch head coach some day.
Ever since Ten Hag managed to treat CR7 at ManU the way he did, I haven't thought much of him as a coach! You saw how ManU played without Ronaldo in the following years and the same would have happened to Leverkusen. So I don't know what they were thinking when they brought him in to replace the great Xabi Alonso.
In his defense, I have to admit that the team has almost completely changed and there are hardly any players left who played under Xabi. But even with Xabi Alonso's squad, Ten Hag would certainly not have made it into the top 3 in the Bundesliga.