Reznikov would be transferred to another ministerial job and replaced by Kyrylo Budanov, head of the military intelligence agency, said David Arakhamia, a close ally of President Volodymyr Zelenskiy on Sunday,
"War dictates changes in personnel policy," Arakhamia said on the Telegram messaging app.
He said that Ukraine's "force" agencies - like the defence ministry - should not be headed by politicians during wartime, but people with a background in defence or security.
There was no immediate comment from Reznikov, a former lawyer who became defence minister in November 2021, a few months before Russia launched its full-scale invasion on February 24, 2022.
The imminent shakeup was also reported by several Ukrainian media outlets citing sources as well as another lawmaker, Yaroslav Zheleznyak, writing on Telegram.
Budanov, 37, is an enigmatic intelligence operative decorated for his role in classified operations who rapidly rose through the ranks to head up Ukraine's Main Directorate of Intelligence.
Arakhamia said Reznikov would be made minister of strategic industries.
His exit from the defence ministry would be the highest profile government change in a slew of resignations and sackings following a corruption scandal late last month.
The shakeup coincides with Ukrainian fears that Russia is planning a major new offensive this month. Ukraine is planning its own counter-offensive but is waiting on Western supplies of battle tanks and infantry fighting vehicles.
As a wartime defence minister, Reznikov, 56, fostered ties with Western defence officials and helped oversee the receipt of billions of dollars of military aid to help Kyiv fend off the Russian invasion.
Reznikov singled out Ukraine's "de facto" integration into the NATO military alliance as a top priority, even if joining was not immediately possible .
During his tenure as defence minister, he spoke out strongly about wartime corruption, which he said was akin to "marauding".
But in recent weeks his own defence ministry became embroiled in a corruption scandal over an army food contract that envisaged paying vastly inflated prices. It caused a public outcry.
One of his deputy ministers has been fired, and two other senior officials have also since left their posts.
Reznikov hosted a news conference on Sunday afternoon, in which he said Ukraine expected a possible major Russian offensive this month, but that Kyiv had the resources at hand to hold them at bay.
He also said that his ministry's anti-corruption department needed to be overhauled and that it had not done what it was supposed to do.
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