The suit says, with some legal authority behind it, that the open ended nature of the authorization is what was illegal. The SC statute does not allow it, and the US Constitution does not provide for rogue prosecutors who don’t have a boss.
The is no SC statute, if you mean special counsel. It is entirely within rules set by the DOJ unto itself. The DOJ has many ‘task forces’ that employ people from outside the DOJ using the same kind of rules.
Even should a court find merit in Manafort’s arguments, his case could just be handed off to a normal US Attorney.