By recusing, has not Session freed himself up to acquire an outside investigator (prosecutor) who will coordinate with Horowitz.
In essence, Sessions has done an end-around from any potential blockages that a conventional special prosecutor system would incur.
The supposed "investigator" is not outside the DOJ. The investigator has worked for the DOJ for a long time. The investigator is "outside" of DC.
Since Sessions is recused on these matters, this investigator would answer to Rosenstein.