For the sake of argument, I propose to take as givens:
- as you suggested, there is not a great deal of variation between the in-game decisions of managers (I don't disagree FWIW, but I really don't have a strong opinion on this particular point).
- I claim that decisions about who to use in the bullpen is impacted by roster decisions regarding the bullpen (many Managers exercise decisive influence on some particular bullpen roster decisions).
- I claim that decisions made regarding the bullpen have a disproportionate impact on high-leverage situations and thus have significant (that is, at least not insignificant) impact on game outcomes.
If the above assumptions are correct, I think managers influence on bullpens would have a significant (again, at least not insignificant) impact on game outcomes. I think the *best* managers in terms of managing bullpens (again, including marginal roster/health selection/management as well as in-game decisions) can influence 5+ wins a season, I would not be surprised at 10, versus the *worst* managers. This is a hypothesis that I have stated for years that some strongly disagree with.
If anybody has stumbled across relevant research on the above, I would appreciate seeing it.