It will not be Xavi, it will be Xabi. They will be completely different people. Also Erik Ten Hag has a very good career history with Ajax. This career history took him to the Premier League. However, his failure in the Premier League is talked about more. There is no doubt that he is a good coach, but we do not know if he can fill Xabi's void in Leverkusen. Because what Xabi did in Leverkusen was a great success. He created a champion team from the young players and the players he recently transferred.
I'm not a big believer in Ten Hag's skills. He might have done well in Eredivisie, but that's a different level from the Premier League or Bundesliga.
I don't even blame him for Man United's poor performance, he's actually not looking that bad compared to United's current manager (Ruben Amorim) who ended the season in the worst league position since the 70s (if I remember correctly). But what I disliked about him when he was leading United is that he didn't seem to be doing much to improve the team's performance. Is like he was carrying on with the same tactic, somehow expecting different results. A good manager should be able to adapt and be proactive.
Overall, I think it will be hard for Leverkusen to either win the league or even finish in the 2nd place next season. I predict Bayern to defend the title and expect Dortmund to be the top contender, if they play as good as they did at the end of the season.