If Putin decides to invade, he will face a reasonably competent Ukrainian military, irregular patriotic forces, and some terrorists. The ensuing occupation will not be a picnic. Could it turn into a new Afghanistan?
No, due to geography and topography. Afghanistan has lots of mountains and poor roads; Ukraine is flat. The Taliban could use Pakistan as a refuge; there is no such refuge for a Ukrainian resistance force. Afghanistan was remote from Russia; Ukraine is next door, and its culture is easy to understand. As a result, the war and the occupation would resemble a much larger version of Chechnya, not the Soviet and American failures in Afghanistan.
Which tells you why the Russians should be concerned. Terrorist strikes within the Russian heartland will be a huge problem for Putin, and leaving Ukraine as a cinder won’t exactly do wonders for the Russian economy. The Pottery Barn rule applies here for the Russians, just as it did for America in Iraq.