I don't think anyone would question that a change of manager can have a huge positive impact on a team, almost immediately. You don't need Tuchel to show that, there's no end of examples. The thing is, there's also no end of examples of the opposite happening, or things basically continuing the same way with the same problems after the managerial change. So that doesn't actually make a general argument that things improve if you change the manager. There's always a downside to changing the manager, in terms of disruption, money, change of course that might not fit the squad and organisation you've built - you lose the benefits of continuity, such as they may be. YOu'd better be reasonably sure the advantages are big enough to outweigh that, if you're going to take that step. Which clubs usually only are when results dip below reasonable expectation for an extended period and they lose confidence things will get better. Particularly when you're in the middle of a season. If you're going to make a change because you don't think your man is the right solution to maximise your potential going forward, that's the sort of decision you take between seasons, not in the middle of one.