At Newcastle it is not as important who is the COACH when players come and go. We roughly know the system, its not some big secret. The coach tells the scouts what type of players they want, and then the scouts go and find them options. The coach then assesses the options and provides a pecking order. The club then makes a business call and attempts to sign players from the list. At least that is how it looks from the outside.
The coach more than likely points them in the direction at times which nationalities and types they would like. So under Rafa you'd expect him to guide the scouts towards Spain or players he knows like Manquillo or Joselu. Standard. You'd also expect defensively sound players. Bruce we will have to see, as he hasn't really had the chance yet. January is largely irrelevant bar a few loans. Looking at that he will also want players he knows like Bentaleb, Rose (English developed). The summer will reveal more and we will see next season what Bruce can do as he starts to change the make up of the side to suit his own needs.
So yeah the manager is partly responsible for the players who come and go, but its probably not worth getting too hung up on the transfer business done. Its a team effort, one hamstrung by the owner. The managers work under the same system though so it is comparable. One which has largely been poor from all in recent times, including Rafa/Bruce or any other manager. We can't pretend otherwise. Pardew had a **** budget but we bought far better players in attacking roles. Rafa being a defensive coach in nature, bought much better players at the back. It appeared Rafa was given a bit more freedom than most when initially building his side, but maybe that was just skewed by relegation.