The CM's in Tuchel's system are in my opinion the most important.
People bang on about how our midfield two cannot provide enough in the final third, being goals or assists. I think this system prevents, or limits them from doing so as it will always be a trade off. Kovacic, Kante, and Jorginho make the system tick because they are very disciplined in their possession and position. Adding a creative midfielder in there is going to create an unbalance in that midfield.