Well, in every sport, the final responsible are the players. I think that a manager doesn't have more than 20% of weight in a team. But that 20% could mark the difference in a Win or a Lose, and mostly in a very competitive league as the MLB. In baseball, before, during and after the game, there are a lot of situations to consider in order to maximize that 20%. Apply a tactic(s)/decision(s) for a certain situation is not trivial, in some cases could be obvious and in some are not; but a decision has to be made and certainly will impact for good or bad in the final result. In the real end, the first responsible if something goes wrong is the manager. They have to live with that since a different move(s)/tactic(s) always can be called if something goes wrong, and they are usually very well paid in order to make a lot of more good than bad calls.