Conor McGregor will no longer headline UFC 200 on July 9, leaving the future of the biggest star in mixed martial arts in doubt. Following a cryptic tweet that indicated he was retiring, UFC pulled McGregor from the event that was expected to become the No.1 pay-per-view event in MMA history.

Considering the increasing popularity of both UFC and McGregor, few, if any, could have foreseen the events of Tuesday. Less than two months ago, McGregor headlined UFC 196, which UFC president Dana White said was the biggest PPV in company history. McGregor suffered his first ever loss in UFC to Nate Diaz at the event, but he was granted a rematch at UFC 200, and the PPV was expected to outdraw the 1.5 million buys that were generated on March 5.

On Tuesday, White announced on ESPN's "SportsCenter" that McGregor was pulled from the card when the 27-year-old refused to travel to Las Vegas this week to promote the fight. Among other things, the promotion includes a press conference appearance and a commercial, which will cost UFC $10 million. White insisted that he remains on good terms with McGregor, and doesn’t know if the fighter is fully committed to retirement.

“He’s in Iceland training and he felt leaving right now would hurt his training and getting ready for this fight," White said. "But every other fighter on the card was coming. I get accused of coddling Conor all the time, but at the end of the day I respect Conor … but it doesn’t make you exempt for showing up for the press conferences and all the promotional stuff that we have to do. We spend a lot of money with this stuff, and you have to do it, man."

There’s been much speculation that McGregor's abrupt retirement involves more than just his reluctance to take part in UFC promotional events. One report claims that UFC was unwilling to pay McGregor the $10 million he wanted for the fight. 

What the future holds for McGregor remains a mystery. The Dublin native and his camp have been conspicuously quiet since his now infamous tweet on Tuesday, but most in the know find it hard to believe that he won’t fight again.

Most fighters, both successful and unsuccessful, traditionally have a hard enough time walking away for good. McGregor has only been in UFC for three years, but his rise to stardom was almost immediate, and he’s doing things that have never been done before. He's also leaving a great deal of money on the table.

McGregor made history when he was guaranteed a $1 million purse for fighting Jose Aldo at UFC 194 in December, and Fox Sports estimates that he could have made as much as $8 million because of PPV revenue. With a 7-0 record in UFC before his loss to Diaz, McGregor became a household name, arguably surpassing Ronda Rousey as the sport’s No.1 star.

Less than 24 hours after announcing his retirement on Twitter, McGregor’s tweet was retweeted 150,000 times. According to ESPN’s Darren Rovell, Kobe Bryant’s retirement poem was the most retweeted athlete tweet of 2015 with 130,000 retweets.

Prior to UFC 196, McGregor was one of the most sport's most dominant fighters in recent years. He needed just 13 seconds to win the featherweight belt from Aldo, who hadn’t been defeated in a decade. Six of his victories came via knockout, and none of those fights lasted beyond the second round.

But McGregor’s success inside the cage is just part of his appeal. The irony of McGregor being pulled from UFC 200 for not wanting to promote the fight is that he's one of the best promoters the company has ever had. Some fighters are disliked because of their confidence and brashness, but McGregor’s charisma has endeared him to die-hard and casual MMA fans alike. His trash talking and swagger have brought plenty of new fans to the sport, and injected renewed enthusiasm from longtime fans.

McGregor has come a long way in just three years. The week before he made his UFC debut on April 6, 2013, McGregor was still cashing welfare checks in his native Ireland. Now, he’s arguably the country's most recognizable sports figure, to the point that Irish legislators were forced to address a petition to put McGregor’s face on their currency.

Rarely one to shy away from the spotlight, McGregor’s loss to Diaz probably isn’t the last we’ve seen of him. There’s even precedent for him to find his way back on the UFC 200 card. Nick Diaz, Nate’s brother, was taken off the UFC 137 card in 2011 because he failed to show up at a press conference, and he ended up in the main event against a new opponent. That isn't expected to happen this time around, however, and UFC is actively searching for McGregor’s replacement.

UFC’s ideal replacement for McGregor might be Georges St.-Pierre. The former welterweight champion hasn’t fought since 2013, but he’s seriously considering making a return to the octagon.

"Who's a good replacement for Conor? I guarantee you Georges St.-Pierre's phone is blowing up right now and he's getting a big offer,” former UFC fighter Chael Sonnen said on Facebook. “That's the guy they're gonna go after."

Another option is welterweight champion Robbie Lawler. Lawler won his fifth straight fight at UFC 195 in January, and Diaz’s win over McGregor came at the 170-pound welterweight limit.

White said that in addition to finding a new opponent for Diaz, UFC is looking to add another fight to the UFC 200 card. UFC No.1 pound-for-pound fighter Jon Jones has expressed interest in fighting at the July 9 event, and he could potentially face light heavyweight champion Daniel Cormier in a rematch of their fight from last year. First, Jones must defeat Ovince Saint Preux in the main event of UFC 198 on Saturday.

UFC 200 won’t have trouble selling PPV buys, especially if Jones vs. Cormier is added to the event. The card already includes Jose Aldo vs. Frankie Edgar (interim featherweight title), Miesha Tate vs. Amanda Nunes (women’s bantamweight title) and Cain Velasquez vs. Travis Browne. But there’s no denying that UFC will feel the impact of losing McGregor, at least for the moment.

UFC is more popular than ever, in large part because of their two big stars. McGregor’s last three PPV’s have averaged a reported 1.75 million buys, and the last two PPV’s headlined by Rousey have reportedly sold close to 2 million buys. All five PPV’s generated over 800,000 buys, something no other UFC PPV has done since UFC 168 in 2013.

But with both McGregor and Rousey absent from the upcoming schedule, UFC may struggle to fill the void.