But Manafort's defense team is expected to use Gates, too.
Gates is also admitting to carrying out a vast scheme to hide millions of dollars in foreign bank accounts on Manafort's behalf. Another special counsel prosecutor, Greg Andres, later clarified that prosecutors had "every intention" of calling Gates to the stand.
Prosecutors are trying to craft a narrative through the various vendors, bookkeepers, and accountants who have testified that Manafort, through Gates, directed his own illicit transactions.
When asked who was in charge in the relationship, Manafort's longtime tax accountant Phillip Ayliff told the jury: "Oh - Mr. Manafort".
Prosecutors will turn to Gates for firsthand support of their accusations that Manafort concealed millions of dollars in foreign income and later lied when he applied for bank loans.
Gates, 46, is already facing up to five years in prison under the terms of a plea deal he struck with prosecutors in February when he pleaded guilty to two charges of conspiracy and lying to the Federal Bureau of Investigation.
Trump admits getting dirt on Clinton was focus of meeting with Russians
House of Representatives Foreign Affairs Committee said on CNN's "State of the Union" show. He said he was not aware of Trump Jr. facing any legal exposure.
"It's kind of leading up to Gates" testimony that, "Yep, this is what he was telling me to do, '" the former US prosecutor said. Downing made the comment in court.
Attorneys representing Manafort have built their defense argument around Gates, painting him as the true culprit behind their client's alleged crimes and asserting that Manafort's only mistake was "placing his trust in the wrong person", referring to Gates.
During opening arguments last week, the defense team made it clear they intend to blame Gates, who handled some day-to-day business operations for Manafort, for numerous alleged reporting deficiencies Manafort is charged with.
Testimony from Mr Manafort's tax preparer continued on Monday.
Under cross examination, she said at the time she believed Manafort was directing Gates' actions and "knew what was going on".
The testimony from Gates, a former adviser to Donald Trump, comes after reaching a plea deal with special counsel Robert Mueller earlier this year to testify against his former partner in a lucrative global political consulting firm.
The trial is set to resume Monday afternoon in federal court in Alexandria, Virginia.