Do you think managers matters in a club? Ofcourse yes, appoints a confident manager and you will see the differences that follows but appoints a mediocre manager, you will also spot the changes in a particular club. Managers are essential because they're in full control of the game and clubs performance.
I used to feel that if a club had world-class players, any coach could manage that club and get good results because the players knew what to do on the pitch to win a game, but after I had seen how different coaches had failed to get good results in clubs that had world-class players, I agreed with you that getting a good manager is very important for a club that has world-class players to flourish and win matches easily.