UFC lightweight champion Khabib Nurmagomedov officially retired from the sport of MMA on Saturday night.

The Russian superstar announced his decision following his second-round submission victory over Justin Gaethje in the main event of UFC 254.

A visibly emotional Khabib revealed that he had promised his mother that he would not fight on following the passing of his father and coach Abdulmanap Nurmagomedov from COVID-19 complications earlier this year.

Khabib walks away from the sport with an unblemished record of 29-0 and with a future spot in the UFC Hall of Fame a mere formality.

However, for as much as "The Eagle" accomplished in his career, he departs the UFC with several high-profile fights left unfulfilled.

Here are just five of those potential bouts.

1) A rematch with Conor McGregor

This had to be first on this list. Khabib and McGregor faced off in the most lucrative fight in UFC history when they met back in October 2018 at UFC 229. Their clash for the UFC lightweight championship in Las Vegas generated an eye-watering 2.4 million pay-per-view buys, smashing all previous records.

p1elfo4kp8qiri2arp21rns1mi9h.jpg

Naturally, the UFC - and McGregor - wanted to repeat the formula and cash in on what would have been a huge rematch. Khabib, who won their initial bout via fourth-round submission, was far less keen though.

It is no secret that Nurmagomedov did not care for McGregor personally - and had publically vowed that he would never face McGregor. Whether this issue could have been overcome is now a moot point. Khabib vs McGregor 2 looks destined to remain the rematch that got away.

2) Tony Ferguson

Speaking of fights that seemed destined never to happen, the bout between Tony Ferguson was booked by UFC on no less than five times. On each occasion, for a variety of reasons, the fight did not take place. 

Once the most anticipated bout in all of UFC, the fight lost some of its appeal when Ferguson was upset by Gaethje in May of this year. However, there is no doubt that a clash between Khabib and Ferguson would have been quite the spectacle. It is a shame that it will now never become a reality.

p1elfoav7ib4r1fll1ocnju21f2uj.jpg

3) Michael Chandler

A former three-time Bellator MMA lightweight champion, Michael Chandler made the switch to the UFC earlier this year. The 34-year-old even served as a back-up for the UFC 254 main event and would have stepped in if either Khabib or Gaethje had been unable to compete on Fight Island.

Given Khabib's dominance over the current crop of UFC lightweights, the addition of Chandler to the mix would could have led to an intriguing showdown. As it is, Chandler will now be keen to get a shot at the championship that will be vacated by Khabib following his retirement.

4) Floyd Mayweather

Khabib became an overnight mainstream sports star when he defeated McGregor at UFC 229. Nurmagomedov's newfound fame was not lost on the ever-opportunistic Floyd Mayweather, who immediately began to campaign for a fight with "The Eagle."

Such a fight was never particularly likely - and is even less so now. If Khabib does choose to end his retirement, it is unlikely to be for a circus-style meeting with Mayweather.

p1elfoc6hrsuv1ujnn308f31djjl.jpg

5) Georges St. Pierre

Now, if there ever was an opponent that might tempt Khabib to compete again, then former UFC welterweight champion St. Pierre is that man. The respect between the pair is well documented and a fight between the pair would be the very definition of a dream match. St. Pierre has not fought since November 2017, but was reportedly willing to make a comeback to face Khabib.

Nurmagomedov's retirement likely ends the competitive careers of both men now that the fight is off the table.

Throughout his UFC run, Khabib gave the MMA world some truly memorable nights. The above list, though, shows that there is plenty more that we could have seen if circumstances had allowed.