I guess he will be an OK hire. He shouldn't have to deal with all the prima donnas he had in Boston. Just a bunch of extras and chorus girls.
My issue is, what is the big hurry? They interview two guys over two days and make their decision? When a business hires a upper level leadership position, it takes months, and they interview at least a half dozen candidates.
As far as whether a manager can make a big difference, I would come on the yes side of that. Not so much in game decisions. Maybe a genius can gain a game or two based on replacing the pitcher at the exact right time. Chances are the new pitcher that comes in will do no better or worse than the guy he replaced.
The difference is in talent evaluation. A good manager can see quickly that a player doesn't have it. In working with the GM and front office, they can get the better players. The GM can work to bring them in, but the manager makes the decisions on playing them or getting them out. The best example here was the Hargrove-Hart tandem. They didn't like each other, but worked well together.