He said testimony from Richard Gates, Trump's deputy campaign manager who pled guilty on Friday to conspiracy against the United States and lying to investigators, will be a key moment.
If he says Manafort "was acting with Trump's blessings" then it's the "end of his presidency".
Prosecutors allege that Manafort, with Gates' assistance, laundered more than $30 million (£21.36 million) and duped banks into lending money. It says the pair used funds from secret offshore accounts to enjoy a life of luxury.
Manafort has insisted he is not guilty and maintains his innocence but Gates' plea increases pressure on him to also seek a deal with Mueller, reports Reuters.
While it was not clear what Gates might be able to reveal to investigators, he was on Trump's campaign team when his then-boss Manafort attended a meeting in June 2016 at Trump Tower in New York between senior campaign aides and a Russian lawyer.
Mueller, according to sources familiar with the investigation, has taken a keen interest in whether Democrats' emails allegedly hacked by Russian intelligence and made public six days after that meeting were discussed then.
Gates helped run the campaign's day-to-day operations, played a key role at the Republican National Convention where Trump was chosen as the party's nominee and accompanied Trump on campaign flights.
He stayed on in the campaign even after Manafort resigned in August 2016 amid a controversy over cash payments from Ukraine. After Trump's election win, Gates was on Trump's presidential transition team and his inaugural committee.
None of the charges to date against Gates or Manafort have made reference to any connection with Russian meddling in the 2016 election or possible collusion.
Russia has denied the accusations of interference. Trump has said there was no collusion, and has also denied any attempt to obstruct Mueller's probe.
Dean served as White House counsel to Nixon from 1970 to 1973. He was dubbed the "master manipulator" of the Watergate cover-up by the FBI but ultimately cooperated with prosecutors.
He has been highly critical of Trump and his administration, previously referring to a memo by former-FBI Director, James Comey, as "the smoking gun".