The minister, Aivaras Abromavicius, a Lithuanian and one of the foreign technocrats appointed to root out corruption, said that a businessman, Ihor Kononenko, had lobbied to have his loyalists appointed managers of a government-owned ammonia fertilizer company to skim off the profits.

“I don’t want to be a smoke screen for obvious corruption or a marionette for those who want to return control in the old style,” he said.

The United States ambassador, Geoffrey R. Pyatt, posted on Twitter in support of the aggrieved minister, calling him one of the country’s “great champions of reform,” as the gap widened between Ukraine’s oligarchs and a Western-backed, reformist wing of the government.

Standing astride that chasm is Mr. Saakashvili, one of the post-Soviet era’s most contentious and best-known politicians in the region, a graduate of Columbia Law School who came to power in his native Georgia after the bloodless Rose Revolution in 2003. So impressed were Western politicians that Mr. Saakashvili once joked that when he walked through Congress he turned more heads than Britney Spears.

At home in Georgia, though, he was a lightning rod for debate, steering a pro-Western course that culminated in a disastrous war with Russia and an electoral defeat. After a self-imposed exile in Brooklyn, he is now reinventing himself in Ukraine.

Anger over corruption was one of the major issues that animated the protests in Independence Square in Kiev, known as Maidan, leading to the demise of the pro-Russian government in Ukraine. But two years and many proclamations later, the country’s ranking in a standard gauge of government malfeasance, Transparency International’s corruption perception index, has barely budged: Ukraine has moved to No. 130 in 2015, from No. 144 in 2013, in the list.

That is little surprise to most Ukrainians, since the new government of Mr. Poroshenko is padded with people drawn from the same corrupt business circles as the old government.