Last time this happened, McGregor was lining up to fight lightweight champion Rafael dos Anjos. McGregor – fresh from a 30-second knockout of decade-long featherweight champion Jose Aldo – was stepping up in weight to fight for another belt.

What happened next is well known. Dos Anjos got injured 11 days out from the fight, and Nate Diaz stepped in to save the UFC's blushes.

Except Diaz went off script, and won.

McGregor and Diaz fought again just a couple of months later in a widely acclaimed five-rounder, and with the scorecard giving the UFC and McGregor the result they needed the brash Irishman is now back on track for his run at greatness.

Again he is fighting for the lightweight belt, but now against dos Anjos' successor, Eddie Alvarez.

For the first time, the UFC is heading to Madison Square Garden in New York, after laws banning the sport in the state were abolished.

Make no mistake, this is one of the biggest fights for the UFC, not just McGregor and Alvarez.

Aside from Ronda Rousey, McGregor is the only other UFC fighter to captivate the mainstream audience. In a sport that is widely considered too brutal for the "more civilised" boxing aficionado, Rousey and her Irish counterpart have brought an interest previously foreign to the UFC.

The recent $5.3 billion sale of the UFC was in large part inflated by the newfound interest in the sport, hot off the coat-tails of Rousey and McGregor.

And this is exactly why UFC 205 is such a crucial occasion in the history of the organisation.

Since Rousey's submission by Holly Holm in Melbourne last year, the women's bantamweight belt has drawn comparisons to Hogwarts' Defence Against the Dark Arts teachers, as no woman has been able to defend the belt.

Rousey's return on New Years Eve cannot come soon enough as interest is starting to wane in that division.

Meanwhile, McGregor is no longer the untarnished fighter he once was. The loss to Diaz eroded the shine his press conferences convey. McGregor was untouchable.

Now he is not.

Diaz, if you will recall, was not a champion, and never was a champion. It was a bad loss for the image of the fighter looking to replicate the persona of recently retired boxing legend Floyd Mayweather jr.

Now, McGregor comes up against Alvarez who himself is looking to secure his position in the upper echelons of UFC champions.

Alvarez represents the UFC of old. An organisation that billed itself on putting together the best fights at all times.

There were always big characters, but no one was above the UFC.

Compared to boxing, where fights could be promoted and postulated for years on end without any quick resolution in the ring (think Mayweather v Manny Pacquiao, and now Gennady Golovkin v Canelo Alvarez), the UFC always had the top ranked fighters coming up against each other in quick succession.

McGregor has changed that. He has brought a persona to the UFC that rivals the trash talk of boxing, but while succeeding in transcending the niche the purity of the UFC's ideals has been lost.

Which is where Alvarez comes in. He said in the lead up to the fight that the UFC should reward the top ranked fighters in the division who have earned their title shots, "not the guys who have a funny accent and sell tickets".

He describes this fight as "killing two birds with one stone". "I was getting the easier fight; on top of that, it happened to financially be the best decision," he said.

McGregor sees his chance to prove that he is more than a headline. He still controversially holds the featherweight title, but has not fought Aldo again and is 1-1 with Diaz.

A loss ends the lightweight adventure for McGregor. He will have no choice but to return to featherweight, where Aldo should be waiting. That will be a much-billed rematch, but it represents the end of the McGregor era.

But the loss would do more damage for the UFC than McGregor.

If he loses, the UFC loses their second-most marketable star. With Rousey already saying she doesn't see a long-term future in the sport, the UFC is banking, yet again, on the Irishman delivering.

Against a fighter of the calibre of Eddie Alvarez, it's a big risk.