Absent that, the only other way to appoint someone is to get senate confirmation via the appointments clause. The president didn't seek this confirmation for the special prosecutor.
This is a very important check on the president's power.
The current president basically hired a private attorney without authorization and sic'd him on his political enemy.
EDIT: Those of you downvoting - why are you against the rule of law? The law on this is very clear, and this issue about this special prosecutor has been known for years.