However, Ten Hag first season with Leverkusen may be difficult because most of the key players will leave before the next season. If Ten Hag wants to bring his former player to Leverkusen, I think he should help Manchester United by signing Onana as their goalkeeper. Manchester United would love it if he can get Onana out of the club, as they are going to sell him after failing to win the Europa League.
Leverkusen are not a very rich club. So I don't expect Leverkusen to spend a lot of money to buy a player. Leverkusen may have found a worthy replacement for Xabi Alonso. Ten Hag is a good enough coach. But it will be difficult for Leverkusen to build a strong squad capable of winning titles.
Manchester United are interested in selling Onana. However, Leverkusen will not be interested in spending a lot of money for Onana. However, Leverkusen may focus on strengthening their attack. They do not need to change goalkeepers. Leverkusen's current goalkeeper Lukas Hradecky is a good enough goalkeeper.