It's very hard to quantify how much of an impact a manager has compared to structure etc.
There's no doubt Brighton are a fantastically run club these days, and I don't doubt the role that Bloom and the backroom staff and structure play.
At the same time I think some people are underestimating the difference a top coach / manager can make as well.
I look at Klopp at Liverpool. Do people think the genius there was Edwards and the transfer committee, or Klopp getting the best out of all those players? A very good coach / manager can turn the perception of a club on its head, from a club buying the likes of Balotelli, Sakho, Lambert, Markovic, Benteke etc deemed utter failures to a club signing Robertson, Mane, Wijnaldum deemed massive successes.
I think we are about to see a similar thing happening at Spurs, although I certainly don't think they will ever reach Liverpool heights, but I think Spurs would be a lot happier with 4th or 5th under the positivity of Ange than the same result under the negativity under Conte.
If the structure is there, then that's great, there's a tone already set at the club. If not, then the manager has to come in and set the tone, almost lift that club, lift that support, and carry it with them, like Klopp did. As much as Utd fans might not want to hear it, I'm just not sure ETH has ever had the personality to do that, and these days, players buy into personalities as much as anything else. Tactics are all well and good, but if your players aren't willing to run through brick walls for you, then if you come across another team that will, you'll get found out. Dutch fans in here warned everyone that ETH is a good coach, but 1 thing that was levelled at him in Holland was his accent / sort of monotone voice and the way he portrayed himself in interviews.
Sometimes that's all it takes for a manager. I look at the stick Emery took for 'Good Ebening', and a very highly rated coach is then demeaned, made a bit of a laughing stock in the media, that filters through to the players and it starts to unravel.
Now, whether De Zebri has that personality that could come in and lift Utd like I mentioned before, I'm not sure, but I think I'd bet on him more than ETH at this stage. I'd certainly take him at Arsenal in a heartbeat if Arteta doesn't work out. By the way, I also think Potter is a very good coach who was set up to fail at Chelsea. My main criticism of him is that he thought it was a good idea to take that job in the first place. Again though, I'm unsure if he could lift a club the way a Klopp can.