Tuchel is undoutbedly a good coach. I argued quite a lot that he's not a great coach when most were hailing him as such, and I stand by it, but whatever the case he clearly doesn't have control over what's happening there. How can he build the team he envisions if he can't convince the owner to buy into it? He couldn't do it in my opinion with Abramovich and the execs back then and now this new owner has stormed in like a bull in a china shop and it's an incoherent recruitment "plan" once again.
He needs to assert himself and demand the money be spent on the players he wants and not just opportunistically on any good player that becomes available. Convincing the recruitment team and DoF etc., of what your ideas is part of being a great coach also.
It's also possible that he doesn't have a great plan or vision for what he wants to build at Chelsea, I would say very often that's what separates a good coach from the great ones.
