At the risk of starting the mother of all flame wars by accident (I’m American, I genuinely don’t know the answer to this and what might happen if I ask), how much difference is there between the Ukrainian language and Russian? They seem to be very closely related from what little I know.
@Bison answered the linguistics fairly well, but I'll add this. I just got back from Ukraine to the states and in Lviv most everyone's first language is Ukrainian and they speak Ukrainian in most situations, that being said, everyone understands and many people, including many young people, speak Russian fluently or near fluently. This includes very patriotic people with heavy Ukrainian roots. However, in Lviv you'll only hear Russian being spoken on TV or by people from Kyiv or some other parts of Ukraine. I spent a weekend in Kyiv (most of my stay was spent studying in Lviv or Ivano-Frankivsk oblast) I do not speak any Russian, so I just spoke Ukrainian to everyone in Kyiv. I was lucky to hear Surzhnyk (a mix of Russian vocab and Ukrainian grammar) and only saw Ukrainian in menus and signs. Mostly everyone just spoke straight up Russian (without a Russian 'moscow' accent to be fair). Nobody I spoke to spoke anything other than English or Russian, or at least, that is what they spoke with me. Most people in Ukraine speak Russian as their first or primary language. However, this is changing and from what I have heard from my Ukrainian friends who are from further east to Dnipro and Kharkiv, or have studied there, Ukrainian is becoming more common, especially in public, which is true to an extent even in Kyiv. For example in Kyiv it isn't uncommon to be handed a Ukrainian menu by a waiter/waitress who doesn't speak any Ukrainian at all, in fact that was my expierance in most cases.
This is not surprising, considering that in Ukraine, language and politics are not connected as much as people believe they are. Rather, there are fundamental philosophical differences between regional mentalities and attitudes among most people. That being said, most Ukrainians in the East wished to remain part of Ukraine and be Ukrainian. In Kyiv, where nobody speaks Ukrainian, patriotism is in fashion. In cities like Dnipro, Odesa, and Kharkiv there were solid efforts to try and create terrorist states like in Donetsk and Luhansk instigated by FSB and terrorists that were put down, for the most part, without serious government intervention. Rather locals, and some more reform minded oligarchs, went out of there way to prevent such attacks. Russia itself was unable to project power into those regions and cities without committing to a full scale conventional conflict and so, have refrained from doing so for obvious political, economic, and military reasons.
Edit: To answer OP's question, is it theoretically plausible? In military terms, yes. However, the issue is that those people did not, nor do they want to secede. The efforts in those areas, which frankly, include damn near all of Ukraine, excluding the existing terrorist occupied areas, never had a strong anti-Ukrainian bent and still do not. Just because they speak Russian does not mean they 1. identify themselves with Russia and therefore against Ukraine 2. support or agree with the Putin regime, which is not as popular as polling in Russia would suggest. There is still a daily Kyiv-Moscow train. In Ukraine and especially Kyiv you hear people criticizing Putin in Russian, occasionally, even with Russian accents. In 2014 to have those ares "secede" would require a full on conventional Russian land invasion.