He is another number 10 who can't play in midfield, a profile that is obsolete. Gonna say he won't live up to the initial expectations, at Chelsea or any other club, because he is very stationary and slow even if his technique is good.
We need to be wary of Wirtz for the same reason + his injury record, although I haven't watched him and don't know if he can play at CM.
Wirtz can play CM no problem. Will probably play it now as he's returning, as there's no CAM in Xabi Alonso's 3-4-3 flat formation, which is big on width, with both LM/RM and LW/RW occupying 4 spots, with LB/RBs playing the LM/RM roles. So the only place left centrally in that formation is one of the two CMs. Will probably be paired up with a more defensive minded CM like Andrich. And he already played that role in the two friendlies since returning (one last month and the other one just a few days ago), played 45 minutes each, and scored a goal in both of them.
If Xavi goes the 'De Jong becoming Busquets successor' route (which is reportedly happening), and we go about signing a progressive CM instead of a natural CDM this summer, Wirtz would actually be the player I'd want us to sign the most. He and Pedri together in our midfield would be a joy to watch, that's for sure. Plus the guy is seemingly a huge Barca fan ever since childhood as well, can't go wrong with that. Who knows, he might even be ready to push his way out or at least put some real pressure on Leverkusen to let him go on the cheap(er) side, if we come for him.