I am a believer that a Major League manager probably isn't going to make that much of a difference with a veteran player. Those guys skills are probably going to remain what they are. Yeah, there are that one in a thousand situations where someone goes Jose Bautista or Sammy Sosa, but by and large, the veteran types stay what they are.
Managers do make a difference, but how much? I say roughly 5-10 games over a season, assuming his decisions don't directly lead to the injury of a star pitcher. I know that some people will say that number isn't high enough, but compare that to players and you are talking a bonafide All-Star player to one of the best seasons ever versus a "replacement player". That is a rather large difference.