If that were true, I don't think we would have named a 29-year-old former player as a manager for the rest of the season - whose results, by the way, were roughly the same as Mourinho's.
It doesn't mean all managers are the same or that you can swap one for the other without noticing a difference. It means that 30-40 years ago, to make it to the top, you had to have a great tactical mind: Sacchi, Cruyff, Lobanovsky, even lesser known manager like Goethals or even Boskov were working with a handful of assistants and were focused on gaining a tactical advantage on the opposition because, contrary to popular belief, most leagues were a lot more competitive back in the day.
Now when you have a squad like Bayern's or Real's, you don't need to be a tactical mastermind to win trophies. You need a solid system and, more importantly, you need to get players on board. For the finer details, you have a team of specialists. You think Zidane takes care of physical training? I know he doesn't. He gives some broad indications on when he wants his team to peak physically and someone else does the work.
That's why Buvac says Klopp's nothing without him, because he worked the finer details.
It's also interesting to note that Mourinho got some great results as long as the players bought into his ideas. Then match fatigue led to a few poor performances, his comments in the media apparently led to a poor dressing room atmosphere which, in turn, led to the Zagreb game.