The only way for Nurmagomedov (or anyone else) to "make Ferguson irrelevant" is by eliminating Ferguson.
Even though it's true that Ferguson has lost wars in his life, he is currently riding a 12-fight winning streak. Furthermore, Ferguson currently has an MMA record of 25-3. I seriously doubt that anyone will think that this war will be easy.
I consider Nurmagomedov vs. Ferguson to be very different from all of the other duels that Khabib has had. Don't get me wrong. McGregor, Poirier, and Rafael dos Anjos are all amazing assassins.
In my opinion, Healy, Barboza, and Iaquinta are all underrated. Having said all of that, however, Ferguson is very different from all of those competitors. Many folks even go as far as to call Ferguson "awkward," "psychotic," "insane," "crazy," "bizarre," "strange," and so many other derogatory terms.
Even if Ferguson is a "weirdo" like everyone says that he is, that doesn't change the fact that he is very dedicated and extremely dangerous. Nurmagomedov vs. Ferguson for the UFC Lightweight World Championship could very well be one of the most dynamic and unique title fights in the entire history of the UFC, perhaps even in the entire history of MMA.