He said there are only three ways someone can become acting attorney general: if they are the deputy attorney general and the president signs an executive order to make them acting attorney general, if they are already at the Justice Department in a job that both required and received Senate confirmation or if the Senate in in recess, they can be a recess appointment.
When asked if he would fire Mueller, as some critics have feared, Trump said, "I could have ended it any time I wanted, [but] I didn't". Senator Susan Collins and Mitt Romney said it should not be impeded in any way.
Trump's relentless attacks on Sessions came even though the Alabama Republican was the first US senator to endorse Trump and despite the fact his crime-fighting agenda and priorities, particularly his hawkish immigration enforcement policies, largely mirrored the president's.
"Trump's actions are being compared with those of U.S. President Richard Nixon in one of the most controversial episodes of the Watergate scandal, the Saturday Night Massacre", said organizers.
The resignation, in a one-page letter to Trump, came one day after Republicans lost control of the House and was the first of several expected post-midterms Cabinet and White House departures.
Over the course of his investigation thus far, Mueller and his team of prosecutors indicted 32 individuals and three Russian businesses on charges ranging from computer hacking to conspiracy and financial crimes.
Trump's relationship with Sessions frayed just weeks into his tenure, when he stepped aside from the Russian Federation investigation.
Mr Mueller could seek to challenge his own firing if he felt Whitaker lacked the good cause required by the special counsel regulation, said Mr Ohlin. We're filing FOIAs demanding answers about Sessions' replacement.
What does this mean for the Mueller probe?The recusal left the investigation in the hands of Rosenstein, who appointed Mueller two months later after Trump fired then-FBI Director James Comey. They also said they will continue to conduct parallel investigations alongside special counsel Robert Mueller's probe, and urging them to preserve all relevant documents.
That's stepping into the unknown, however.
Good cause is a hard thing to show, especially since Mueller's actions were approved by the deputy attorney general, said Jens Ohlin, a professor of criminal law at Cornell Law School.
Those would be half-measures and insurance policies to limit damage.
Mr Whitaker has publicly criticised Mr Mueller's conduct several times since the inquiry began in May past year and taken the side of President Trump, who has frequently...
There's some doubt about whether the president is right, but with the mid-terms behind him he could be itching to settle this Mueller business once and for all.
That nearly certainly wouldn't be the end of this story, but it's the beginning of a new, fraught chapter.