I like to take the piss out of Werner, but his main strength is the quality of runs he makes off the ball, which frees up space for other players, which was demonstrated perfectly in the Havertz goal in the CL Final. Not sure Lukaku would benefit a lot from those runs. Also, Tuchel doesn't have a good record managing target men, apart from his time at BVB when he made Aubameyang a star (although that was in a compeletely different system to what he plays now at Chelsea).
As you said, Lukaku will be extremely good when it comes to holding up play, but when it comes to having space and finishing, I'm not so sure. If he struggled at Olimpico against Roma and Smalling last year, what chance does he have against defences like Liverpool's, City's, United's or even ours if we manage to get Torres or Laporte?