You need the will, the way, the weather and the terrain.
If you do not have control of all of those aspects you will not win.
They do not have the will. Eastern Ukraine would be just as happy being Russian. The locals will not support a bloody conflict.
The Ukrainians do not have the way. Their military could put up a fight, but they will lose in a head to head confrontation with the Russians.
The weather also falls to the Russians. I am sure they have more modern all weather aircraft, night fighting capabilities, and more sophisticated satellite and drone technology.
The terrain is a push.
Logistically, the Russians can replenish their armaments by drawing against their stores. They can get them to the front by rail or air.
When the Ukrainians fire an anti tank missile....that is one less they are going to have.
So, as far as I can see, the Russians win four of five points, and the last point is a push.
The Ukrainians will pull out and either surrender en masse, or beat feet back to the Western part of Ukraine.
Ah yes, Western Ukraine, where the Poles were kicked out after the end of WWII.