It's unrealistic to expect that the manager would have total control, at least not in today's game.
Can Andy Green decide to make Christian Scott, for example, a relief pitcher without the blessing of the front office? Would Mendoza have been able to do that? Will next year's guy, whoever he is? I doubt it.
Going further back, if Davey Johnson had wanted to move Ron Darling to the bullpen, was that decision entirely in his hands, or would he have needed Frank Cashen's approval? I suspect he would have.
What about if Gil Hodges decided to make Gary Gentry a relief pitcher? Maybe in those days a manager could have acted unilaterally.
I suppose we can be pretty certain that there were very few limits on what John McGraw or Connie Mack could do, but we're going back a very long time to their days.