ABU DHABI (AFP) - Dustin Poirier knocked out Conor McGregor in the second round of their UFC lightweight bout in Abu Dhabi on Sunday (Jan 24) to register revenge over the Irish superstar.

American Poirier staggered McGregor with an overhand left and then dropped him to the canvas with a straight right before referee Herb Dean waved it off at 2 minutes and 32 seconds of the second round.

The controversial McGregor, known as "Notorious", was returning to the octagon for the first time in a year.

The second-ranked Poirier, 32, has now put himself in line for a shot at the lightweight title.

"I think this was a title fight - I'm the champion now," said Poirier (27-6) after the UFC 257 fight.

McGregor (22-5) vowed he would be back.

"I have to dust it off and come back and that's what I'll do," the 32-year-old said.

"I'm gutted, it's a tough one to swallow. I just want to go back to the hotel and chill with my kids and regroup."

But he later admitted that his future in the cage uncertain.

"It's hard to take," said the biggest drawcard in mixed martial arts.

"(It's) the highest highs and the lowest lows in this game. I don't know where I'm at, at the minute, to be honest."

McGregor had been an overwhelming favourite to register a winning return after a year away from the octagon.

But Poirier, who lost in 109 seconds to McGregor in their first fight in 2014, used a lower leg kicking strategy to negate his opponent's stand-up style.

"I have no excuse. It was a phenomenal performance from Dustin. It's a bitter pill to swallow," said McGregor, who hobbled onto to stage with help of a crutch and revealed his calf had blown up "like an American football".

The unexpected defeat left many MMA fans wondering what will be next for McGregor, who will face questions about his future in the sport.

The loss will scupper any possible rematch with Russian Khabib Nurmagomedov, the unbeaten UFC lightweight champion who has now retired.

Nurmagomedov's submission victory over McGregor in 2018 set an all-time record for UFC pay-per-view with 2.4 million buys.

The Russian was quick to mock McGregor on social media after Sunday's fight.

"This is what happened, when you change your team, leave the sparring partners who made you a champion and sparring with little kids, far away from reality," tweeted the Russian.

The defeat may mean McGregor now explores other avenues, such as a money-spinning boxing match against the Philippines' great Manny Pacquiao.

McGregor fought retired unbeaten boxing great Floyd Mayweather in 2017, earning an estimated US$30 million (S$39.85 million) in a blockbuster crossover bout.

McGregor deflected questions about the possibility of a fight against eight-weight world champion, saying he was more concerned with healing his damaged leading right leg.

"I'll regroup," said McGregor. "I'll pick myself up, get up off the floor and go again. I'll keep my head high."

Pacquiao had wished McGregor luck on social media ahead of and tweeted afterwards: "Beautiful thing about the fight game, anything can happen! Congrats to @DustinPoirier Huge win! #UFC257."

There was still time though for one last flash of McGregor's old brashness, before he limped away, when he was told of Nurmagomedov's mocking social media posts.

"That's the character of the man for sure, behind the mask," McGregor snapped back.

"If he wants to have his disrespectful comments, come back and let's go again."