In essence what Rosenstein has done is take someone off the street (not someone already a prosecutor) and purport to vest that person with all the awesome power and authority of a United States Attorney. But without having to go through the nomination and confirmation process of an actual US Attorney.
I gather that Rosenstein senses that legally speaking he is on thin ice as to the validity of the Mueller appointment, based on what I heard from him the other day at his talk at the Newseum.
Rosenstein made a passing remark about the legal basis for appointing special counsels and indicated that the legal power for the DOJ to appoint a de facto US Attorney like Mueller derives from the same source by which the Attorney General can appoint a temporary US Attorney when the post is vacant.
But that’s really apples and oranges and shouldn’t get them far with a good judge.