Updated at 3:57 p.m. ET

Paul Manafort's former business partner, Rick Gates, was set to take the witness stand on Monday for what may be the most critical testimony in Manafort's bank and tax fraud trial.

Gates worked as Manafort's right-hand man over the past decade, and prosecutors say he helped Manafort evade taxes and lie to banks to qualify for loans.

There had been some discussion in court on Monday that suggested Gates would be the next witness but prosecutors did not call him as it appeared they might.

Gates pleaded guilty earlier this year as part of an agreement to cooperate with federal prosecutors.

Multiple accountancy and bookkeeping witnesses testified last week that they dealt with Gates in preparing Manafort's financial paperwork. Prosecutors were expected to ask Gates for an inside look at what they say was a scheme by Manafort to use offshore bank accounts, shell companies and wire transfers as to avoid hundreds of thousands of dollars in tax bills.

But Manafort's defense team is expected to use Gates, too. Attorney Thomas Zehnle accused Gates of embezzling money from Manafort's consulting firm and said that Gates was now agreeing to cooperate with prosecutors to cover his tracks.

"Rick Gates had his hand in the cookie jar, and he couldn't take the risk that his boss might find out," Zehnle said.

Juries are typically skeptical of witnesses who enter into agreements with prosecutors, said Sam Buell, a former federal prosecutor and a white-collar law expert at Duke Law School, so he said he was not surprised by the defense's strategy of going after Gates.

Gates hasalready admitted to lying to investigators once.

Buell also said it's very difficult to find a strategy for defending against fraud charges in cases with an extensive paper trail.

"You've got to try to find a way to throw a monkey wrench in," Buell said.

Prosecutors have been preparing for this credibility battle by asking a number of witnesses speak to Manafort's role as a supervisor over Gates.

When asked who was in charge in the relationship, Manafort's longtime tax accountant Phillip Ayliff told the jury: "Oh — Mr. Manafort."

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