https://www.washingtonpost.com/politics/2018/09/24/rod-rosenstein-resignation-would-affect-mueller-probe-ways-you-might-not-expect/?utm_term=.e00a5022184c
"So Francisco, having been confirmed by the Senate to his position, is the assumed next-in-line for the Mueller probe. Unless, that is, he has to recuse himself given his former law firm’s involvement in the Trump campaign (a possibility raised on Twitter by former Office of Government Ethics head Walt Shaub). In that case, the job would probably go to Steven Engel, the assistant attorney general for the Office of Legal Counsel."