Allen H. Weisselberg, who for decades was one of Donald Trump’s most trusted executives, is expected to plead guilty on Thursday to a long-running tax scheme at the former president’s family business — a serious blow to the company that could imperil its chances in an upcoming trial.

The plea deal will allow Weisselberg, who was facing up to 15 years in prison, to spend as little as 100 days behind bars, according to people with knowledge of the matter. And it does not require Weisselberg, the Trump Organization’s longtime chief financial officer, to cooperate with the Manhattan District Attorney’s Office in its broader investigation into Trump, who has not been accused of wrongdoing.

But Weisselberg is expected to admit to all 15 felonies he was charged with and will have to testify about his role in a scheme to avoid paying taxes on lavish corporate perks, the people said. That requirement will put the company at a disadvantage and make Weisselberg a central witness at its trial in October, where it will face many of the same charges.

Weisselberg is not expected to implicate Trump or his family when he takes the stand in the October trial, the people said, and on cross-examination, the company’s lawyers could accuse him of pleading guilty only to spare himself a harsher sentence.

But his testimony — an acknowledgment from one of the Trump Organization’s top executives that he committed the crimes listed in the indictment — will undercut any effort by the company’s lawyers to contend that no crime was committed. The indictment placed Weisselberg at the center of the company’s scheme, and his testimony could enable prosecutors to argue that his admissions go a long way toward proving its broader claims.

Weisselberg, 75, and the Trump Organization were indicted last year by the district attorney’s office and accused of orchestrating a scheme in which some executives were compensated with special off-the-books perks. Weisselberg, the prosecutors said, avoided paying taxes on $1.76 million of his income over the last 15 years.

The investigation into Trump has largely focused on whether he fraudulently inflated the value of his real estate and other assets to obtain loans and other financial benefits. Over the last year, the prosecutors have examined Trump’s annual financial statements, which he provided to banks and insurance companies.