Rosenstein being careful or not, think of the chilling effect Rosenstein being Huber's boss has on Huber.
Sessions is Huber’s boss. The IG and Huber are not investigating anything related to Trump-Russia. They are looking into everything else within the DoJ and FBI; and there is plenty.
Sessions has said Huber has full prosecutorial powers. Any action by Rosenstein to interfere with Huber’s action would be obstruction of justice.