Manafort and his associate Rick Gates were placed under house arrest this week on money-laundering charges linked to an investigation by special counsel Robert Mueller.
The charges relate to allegations that the then Republican presidential candidate's campaign colluded with Russia in the runup to the November 2016 election.
The US Congress is also conducting three separate investigations into the allegations, which have dogged Trump since his election and which he has strongly denied.
Asked who recommended Manafort manage the campaign, Trump said "it was a friend of mine who was a businessman -- a very successful businessman and a good person, and you know, Paul was not there very long."
"I think we found out something about (how) he may be involved with all -- with certain nations, and I don't even know exactly what it was in particular," he told Sharyl Attkison's syndicated television show "Full Measure"
"But there was a point at which we just felt Paul would be better off -- because we don't want to have many potential conflicts," he added in the show which airs in full on Sunday.
Trump said he has no current plans to fire Mueller.
"Well, I hope he's treating everything fairly, and if he is, I'm going to be very happy, because when you talk about innocent, I am truly not involved in any form of collusion with Russia," he said.
"Believe me, that's the last thing I can think of to be involved in."
The president said he was not aware of any plans to be questioned by the special counsel.
"As far as I'm concerned, I haven't been told that we're under investigation, I'm not under investigation."
Manafort and Gates have pleaded not guilty to charges that they allegedly hid millions of dollars gained from work with Ukrainian politician Viktor Yanukovych and his pro-Moscow political party.
A separate plea deal revealed that campaign foreign policy advisor George Papadopoulos had lied about contacts with Russians.
Another advisor, former Navy officer and investment banker Carter Page, admitted he had met with Russian government officials in 2016.
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