Trump appears to be attacking Rosenstein now after he previously told Sessions to get Rosenstein to draft justification for the dismissal of Comey.
Rosenstein is a Trump appointee, and is in charge of the Russian investigation because Sessions had to recuse himself.
Rosenstein is the only person who can fire Mueller, and Rosenstein has indicated that he will not.
Trump is now attacking Rosenstein, which likely means Trump wants to get rid of Rosenstein, in order to get rid of Mueller.
Queue Saturday Night Massacre part deux.