Or he's simply not very good. The likes of Mourinho and Pep are often called cheque book managers but at least with them you know they're buying specific players to play a specific way in a certain system. It looks like Chelsea have just signed players for the sake of it and are going to try to figure out how to play and how to fit them in after. It's not just the new signings either. You've got Jorginho, who to get the best out of you need to play a very specific way, who's been in and out of the side with Lampard chopping and changing how he sets up in midfield and that was the case last season too.
I haven't seen a great deal of Havertz but he doesn't seem to specialise one position. I'm not entirely sure why they've spent a tonne on him and completely neglected their defense.