Brigadier-General Andriy Hnatov was replaced as the battlefield commander in the east by Major General Mykhailo Drapatyi, overall commander of ground forces, who will keep his previous duties.
Hnatov was given a role overseeing training and communications.
In his nightly video address late on Sunday, Zelenskiy said the aim was to strengthen the command of troops in the Donetsk region.
Donetsk, a battlefield since 2014 and one of four provinces Russia claims to have annexed since its 2022 full-scale invasion, has been the main focus of fighting for more than a year.
The Ukrainian military confirmed on Monday that it had withdrawn from the Donetsk region town of Velyka Novosilka, a day after Russia said it had captured it.
Viktor Trehubov, a military spokesman for Ukraine's eastern front, confirmed that Russian troops had entered the town but said fighting continued on the outskirts.
It's been nearly six months of active operations in the Kursk region to defend Ukrainian territory. We are maintaining a buffer zone on the Russian territory to protect our cities of Sumy and Kharkiv from Russian offensives. I am grateful to all our warriors who defend Ukraine's… — Volodymyr Zelenskyy / Володимир Зеленський (@ZelenskyyUa) pic.twitter.com/ci6KMjPDWQJanuary 26, 2025
Russian forces have been slowly but steadily advancing in eastern Ukraine for more than a year in relentless ground combat that has caused massive military losses on both sides.
Ukraine, for its part, has managed to capture and hold a pocket of territory inside Russia over the past six months.
The new eastern commander, Drapatyi, 42, is well respected in the army, where he is credited with stopping a Russian offensive in the northeastern Kharkiv region last year.
Ukrainian forces halted Russia's initial assault on the capital Kyiv after Russia's invasion in 2022 and achieved several big successes recapturing territory during the first year of the war.
But they have largely been on the defensive since the failure of a major counterattack in mid-2023.
Ukrainian analysts estimate Russia captured about 3000 square km of territory last year.
With the war approaching its three-year mark in February, Ukraine is outmanned on the battlefield and its troops are exhausted.
The government tried to address the issue by lowering the mobilisation age to 25 from 27 and introducing tougher rules for those evading the call-up.
But it has so far resisted foreign urgings to lower the mobilisation age further to boost manpower.
Trehubov, the eastern forces spokesman, said that the logistics hub of Pokrovsk in the Donetsk region remained the main Russian target.
About 7000 people are believed to remain inside Pokrovsk, which had about 60,000 residents before the invasion.
"The Pokrovsk direction accounts for at least half of all combat clashes, there is a lot of fighting there. It is the most active direction," Trehubov told Ukrainian TV.
"They are trying to bypass the city, cut the supply lines and drive our troops out of the city."
Ukrainian air defences downed 57 out of 104 drones launched by Russia overnight, the air force said on Monday, with officials saying infrastructure was hit in several regions.
The air force said that 39 drones were "lost," in reference to Ukraine's use of electronic warfare to redirect Russian drones.
The military said Dnipropetrovsk, Sumy, Ivano-Frankivsk and Kyiv regions were affected by the attack.
"Infrastructure facilities, apartment blocks and private houses were damaged. Preliminary reports of no casualties," it said on Telegram.
Governors of Sumy, Ivano-Frankivsk and Dnipropetrovsk regions confirmed that drones hit "critical infrastructure" but gave no further details.
The Russian defence ministry said in a statement its drones struck the infrastructure of Ukrainian military airfields, production workshops and storage facilities for unmanned aerial vehicles in 149 areas.
Reuters cannot independently confirm the statements.
