As I pointed out at the time, Rosenstein "authorized" Mueller to investigate anything.
As long as Rosenstein remains in his position, the swamp isn't going to be drained.
I think the point of Manafort's suit is that Rosenstein did that, and that he cannot do that under both the law and the Constitution.
It's worth recalling that SCOTUS threw out the original special-prosecutor law for two reasons: a) you can't have an investigator not under the control of the Executive; and b) part of the reason why is that the investigation must be supervised and limited in scope (probable cause and those pesky 4th Amendment and due process requirements).
Since Sessions has "recused" himself and Rosenstein is manifestly in cahoots with Mueller, I would think Manafort has good grounds to get his case tossed.