In contrast to the Premier League, the german Bundesliga is easier for Ten Hag and a top 5 finish should definitely be possible. But if he also fails at Leverkusen, it will be clear to everyone that Ten Hag is not a good coach. Even if some important players have now left, the team is still strong enough for the Bundesliga and Xabi would certainly have reached the top 3 with this team.
Ten Hag has a lot to prove at Leverkusen, especially after his struggles at Manchester united. You are right, Bundesliga shouldn't be that difficult compared to English premier league, but the shocking thing today is that he lost 5-1 to Flamingo U20 in a supposed friendly game. Unfortunately, I was not able to see the line up, but losing 5-1 to U20 of another side is literally crazy. This is clearly a poor start if you ask me, though pre season games don't count much, but you have to show that you're ready.
And I'm not sure why Leverkusen hasn't replaced some of the players that left the club. Players like Frimpong and Whirtz are currently out, while xhaka should be leaving anytime soon. ETH needs as many resources as possible, because there is no excuse this time.