That is the same for every coach joining a new club, they start from scratch more or less although Xabi Alonso left a good portion of an intact team behind. It's not like all the players left. Losing Frimpong and Wirtz is tough though.
Losing Frimpong and Wirtz was a major part of Leverkusen with Xabi. Look at the stats of how they played. Frimpong starts a lot as a starter and it's the same with Wirtz. I think Ten Hag will have big expectations and pressure because Xabi has left behind good stats. Ten Hag now just needs to work and make Leverkusen better. The German league is also not as tough as the EPL I think Ten Hag has a great chance to succeed with Leverkusen. In addition to these two, there are 3 more players being sold including Tah, Gustavo Puerta, and Odilon Kossounou.