Here’s one of the places it’s documented that Rosenstein told Mueller that he would leave it up to Mueller if Rosenstein should recuse himself. This apparently happened way back before Mueller was even appointed, which leads one to wonder if this all hasn’t been a setup from the start.
http://dailycaller.com/2017/06/04/russia-probe-may-expand-to-include-sessions-rosenstein/
It is the plan and so far they have executed it with no impediments. Which leads me to wonder if Sessions is in on it. He is suppose to be a great lawyer but this needs to be stopped cold.