One absolutely can given that Russians have historically endured great sacrifice to protect **Russian** soil. So either Russians have changed their attitudes and will no longer make such sacrifices for Russian soil, very very doubtful, or they don't consider the Ukraine to truly be Russian soil requiring such a sacrifice.
In case you haven't noticed, there are already thousands of Russian volunteers fighting in militia units in Donbass. And it keeps ramping up.
That theory seems to fail given the level of active Russian interference in Ukrainian affairs.
I was talking about a popular attitude of Russian citizens, which is not necessarily shared by the Russian government.
Also note that the heavy-handed intervention always seems to be stopping short of full-on invasion (which, let's face it, would have the country steamrolled in matter of weeks if not days). Right now it's arms and munitions supply, artillery strikes across the border where the range allows, and occasional infiltration by the more elite units to lend a hand at hot spots (like Mariupol), but all covert and unofficial. No air support - which would make a really huge difference! - and no Russian tank columns rolling across the border in numbers.
From the "failed state" perspective, this actually makes a great deal of sense - you want to help it fail faster, but don't want to overtax yourself in doing so.