A quarterback that makes very few costly mistakes or turnovers. Game managers rely on a strong defense and rushing attack to win the game.
If a team can win games with a game manager at the quarterback position it is a strong indication of a good all around team.
In 2011 and 2012 (before Colin Kaepernick) the San Francisco 49ers quarterback Alex Smith was often thought of as a good game manager (as opposed to an elite quarterback). He minimized turnovers which plagued his career in previous years, and relied on a great defense and running game with Frank Gore to win.