As reported by Censor.NET citing ZN.UA, this is stated by a graduate student at Oxford University, Samuel Ramani in Washington Post.
On Aug. 9, 2015, a senior Russian general declared that if the Ukrainian military crosses Russia's red line and attempts to recapture the Donbas region of eastern Ukraine, Russia would respond with overwhelming force. This statement reaffirms the Kremlin's official line that Russia needs to hold onto territory it has virtually annexed in the Donbas to ensure the viability of its puppet republics, Donetsk and Luhansk. But is it true? Samuel Ramani has a different opinion.
According to him, Vladimir Putin quite probably regards the takeover of the Donbas territories as temporary, and is evaluating this occupation with a strict cost-benefit analysis. Right now, he has concluded that small military victories in the Donbas generate more than enough political capital in Russia to offset the Russian public's disdain for the hardships of sanctions-induced austerity. Should that assessment change, Putin is very likely to tactically withdraw from the Donbas on his own terms. Putin will not regard this withdrawal as a defeat, as Russia will retain a military force in Crimea that could be used to destabilize Ukraine if it tries to join NATO.
The author observes that today there are at least three factors indicating that Putin may decide to retreat in his war against Ukraine.
Firstly, Russia's military presence in and occupation of the Donbas territory is much less popular among Russian-speaking Ukrainians than Putin initially predicted in 2014. Russia would have a difficult time controlling the Donbas in the long-term because its imperialism is unpopular there. Putin did not expect mass resistance from Russian speakers in eastern Ukraine, but that's what he has gotten.
Secondly, the Ukrainian government has more power than it has used in the conflict thus far, and it could use this influence to force Putin to back down sooner than expected in the Donbas. Kyiv has got several more options for reinforcing its positions. Namely, it should focus on economic interaction with China and the EU.
Thirdly, Putin's long-term goal is to create a "frozen conflict" in the Donbas, a scenario in which active fighting is suspended but ethnic tensions remain and can reignite at any time. It's cheaper and poses more threats for Ukraine. Russia will therefore keep troops on the Ukraine border should it tactically withdraw from the Donbas territories. The implicit threat would be the same as in Moldova: enter NATO and risk war. Meanwhile, if Russia were to end its occupation and create a frozen conflict, it could save some rubles.
The author says that Russia's objective lies in affecting Ukraine's foreign policy. It is easier to do with no further territorial expansion.
Related materials: Russian aggression against Ukraine