They can draft reservists, they can reshuffle command, they can reform logistic and they can even use tactical nukes.
If they do, then you can consider this the third Russian Empire to have collapsed in just over a century.
If Russia was to use a single nuke, it would likely trigger a NATO response. Because as everybody over the age of 8 should know, that would irradiate contamination onto multiple other countries. And I can't think of a faster way to not only trigger a nuclear exchange, but if there is not an exchange a lot of nations in Europe would likely throw off the NPT. Because the only thing protecting most of Europe against nukes is the fact they are under the "Nuclear Umbrella" of the US, UK, and France. If all three refused to retaliate, then odds are nations that are nuclear capable in theory would decide that nobody will protect them and start their own nuke programs.
And that includes Germany, Japan, Canada, South Africa, and Netherlands just to start. Each and every one of them could become a nuclear power very quickly, they only have not because of agreements with nuclear powers to protect them. But any nation using a nuke would cause a lot of nations to throw away the NPT and develop their own program for self-protection.
Latvia, Lithuania, Poland, Slovakia, Hungary, Romania, each and every one of those would be affected by nukes because of the fallout, and would consider that a form of chemical attack. And even China would likely react, as no matter how chummy they have been acting lately, they are still nation strongly opposed to each other and have come to blows between each other multiple times over the decades.
