The idea that the region was Russian before it was Ukrainian is false except in Crimea (where it was Tatar) & maybe the far east, around Donetsk. But even there Russians had been moving in to work in industry in the 19th century.
afraid this is Russian propaganda, apart from Crimea, where Russians forcibly removed most of the Tatars who used to be most of the population.
What is definition of ethnic Ukraine ? There's many definition on that but seems most historian agree the origin of this ethnicities are west of dnieper river. By that definition the Ukrainian Etnics are those that in Western Ukraine.
Present Ukrainian regime also put counter Propaganda that saying those in Ukraine are indentifying themselves as Ethnic Ukraine, including those in East. So it is Ukraine Propaganda against Russian Propaganda which basically both of them are similarly questionable.
For that I see 3 things that really shown what ethinicities they are:
1. What is their mother language,
2. How they are political tendencies (where they vote goes),
3. Where those teritory actually originated from.
By those three definition, the Eastern Ukraine are Russian ethnicities. They talk Russian, Politically they consistently vote for Russian Ethnic President (Yanukovich), and until USSR time their teritory not belong to Ukraine.
If we go to history, then most of the teritory in the black sea coast of that area is belong to Tartar. The Turks say those teritory is Part of Otoman area. That's why what matter is whose that now resides on that teritory.
I'm not going to support either Moscow or Kyiev. In my opinion both of them are equally responsible on this mess. However by looking on their voting patern those in West and East Ukrainian are definitely already on seperate ethnic thinking.
West always said Moscow enforce their will on people in Donbas and Crimea, but same thing can be said with Kyiev now with ethnic Russian in East.
That's why I agree with some assesment in the West (those maps that I put not coming from Russian sources, but Western ones), that ethnic government power division on Ukraine has to be set. Just like what already been done in Yugoslavia. It's the safest way to ensure Ukraine border as it is.
Russia behaviour basically is to counter Yanokovich ousting by Pro Western Ukrainian ethnics. I only point out when there too much ethnics divide, it will become a powder keg, just like what happen in Yugoslavia and Cyprus and in some sense also Syria.
Why Western governments can accept ethnic divide in Bosnia and Kosovo but will not accept same thing in Ukraine ? The Kyiev regime now shown hard to the West that there's no ethnic divide in Ukraine. How they can said those in East is ethnic Ukranian when they talk Russian and vote Russian Ethnics ?
Add:
With Russian Ethnic federal power in East, basically each ethics group govern themselves. It's not pretty solution, but either that, or some area in East will definitely separating from Ukraine. That's what powder keg that waiting, which can be bigger then current rebellion in Donbas.