Speaking in the Oval Office, the president said a pardon was “never discussed, but I wouldn't take it off the table".
Mr Trump also kept up his recent attacks on Mr Mueller's Russia probe, comparing it to the 1950s era of Joseph McCarthy.
"We are in the McCarthy era. This is no better than McCarthy. And that was a bad situation for the country. But this is where we are. And it’s a terrible thing,” Mr Trump said.
Mr Trump, speaking as part of an interview with The New York Post, went on to describe Manafort, as well as long-time confidant Roger Stone and right-wing commentator Jerome Corsi as “actually very brave.” All three of said to be of interest to Mr Mueller's investigation into Russian election meddling and possible collusion with Trump campaign officials
Prosecutors for the special counsel said in a court filing on Monday that Manafort had lied to them, breaching a plea agreement signed after he was found guilty of eight counts of financial crimes in August. Those charges related to political lobbying work in Ukraine..
In its court filings, the special counsel wrote “Manafort committed federal crimes by lying to the Federal Bureau of Investigation and the Special Counsel’s Office on a variety of subject matters.”
Manafort said in the same filing that he disagreed with the special counsel’s allegation. His legal team said they believed he had given truthful answers to all questions.
Mr Trump's interview followed a report in The New York Times that Manafort's attorney has been briefing Mr Trump's lawyers about what Manafort has been telling federal investigators. Such conversations are unusual but not illegal.
Manafort was likely facing about 10 years in prison for the eight guilty counts in the Virginia case, sentencing experts have said.
Under the terms of the plea agreement, Manafort plead guilty in September to two charges of conspiracy.
The president has been vocal in his support for Manafort, lauding him as a “very good person” during the Virginia trial.
Meanwhile, key Democrats have rejected the notion Mr Trump may pardon his former campaign chairman. Mark Warner, the top Democrat on the Senate intelligence committee, described the potential move as a “blatant and unacceptable abuse of power” in a tweet following the president’s interview.
“The pardon power is not the President’s personal tool for protecting himself and his friends,” the senator wrote.
Reuters contributed to this report.
Join our new commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies