Especially in American football.
Of course, it depends on what you mean by "great coaches." Coaches usually gain that reputation by achieving championships, and for that, one needs pretty good talent.
But a coach can make a huge difference in raising the ceiling for a given roster and keeping it off the floor. Coaches create and sustain a community commitment to discipline and effort. They put the players they have into positions encouraging success. They create schemes reflecting their talent, responding to opponents' talent and tendencies and creating positive odds for success. Perhaps most important, they create structures encouraging success which convince players to buy in and believe.
A coach cannot raise a team above its ceiling, but he can help it discover a higher ceiling than was realized and to play damn close to it damn near all the time.
I actually believe pretty strongly about this. It isn't true of all sports--notably the NBA. But in football, coaches make an enormous difference, and teams are almost incapable of succeeding without good coaching.
I do notice that not all fans (or players) feel this way. It's especially noticeable among Ram fans who wonder whom to hold responsible for all the years of failure. I am one of those who believes that coaches make players better or worse.