Joe Fan
10,000+ Posts
I won't sit and wait for Rosenstein to recuse himself.
He wrote the memo that Comey should be fired upon which, at least in part, Trump based his decision to fire Comey.
To the extent Mueller is/was looking into whether Trump firing Comey was 'obstruction of justice' (hint: it's not) then Rosenstein himself became a potential fact witness. This is an automatic recusal at DOJ.