It's the same type of error Pep made when he got Zlatan. Tuchel had a quick, hard-working player in Werner, who is a professional and very mobile and with pretty awesome stamina, but he lacks end product in some areas, mainly finishing and being a solid target man to battle with defenders.
So he got Lukaku, thinking he will fix what was missing with Werner. But he didn't consider that Lukaku while improving those areas will expose the areas that someone like Werner provided very well for the team.
In the end, it's a tradeoff. For a press-based tactic, I'm not convinced someone like Luaku is overall better than someone like Werner, or someone like Diego Jota. In some ways, no doubt he is, but in others he isn't.
It's not a coincidence both Zlatan and Lukaku shine in Italian football at teams playing very much a conservative type of football where the star 9 is exempt from doing too much legwork with pressing and such.
Lukaku and Zlatan are just not do-it-all humble types to be shaped by the likes of Tuchel and Pep. You either take the whole package, with the good and the bad, both in generous quantities, or you stay away. They want the team to play for them.