Ultimate Fighting Championship star Conor McGregor on Monday called President Trump "quite possibly" the greatest president of all time while wishing the U.S. a happy Martin Luther King Jr. Day.

"Phenomenal President," McGregor tweeted to his 8 million followers in response to a tweet from Trump. McGregor included an emoji of a goat, which in sports circles means "greatest of all time."

"Most certainly one of them anyway, as he sits atop the shoulders of many amazing giants that came before him," McGregor, 31, added. "No easy feet. Early stages of term also. Incredible. Congrats and Happy Martin Luther King Jr. day America."

The tweet was responding to a Trump tweet earlier that day that was celebrating his three years in office while touting low unemployment numbers for African Americans in recognizing Martin Luther King Jr. Day.

"It was exactly three years ago today, January 20, 2017, that I was sworn into office," Trump wrote to his 73 million followers. "So appropriate that today is also MLK jr DAY. African-American Unemployment is the LOWEST in the history of our Country, by far. Also, best Poverty, Youth, and Employment numbers, ever. Great!"

Unemployment in the black community is at 5.9 percent, a record low, according to the latest figures ending in December. For context, the black unemployment rate in December 1981 was 20.9 percent.

In 2015, McGregor had a different take on then-presidential candidate Trump, saying he should "shut his big, fat mouth" after Trump commented on a loss by fellow UFC fighter Rounda Rousey.

"Glad to see @RondaRousey lost her championship fight last night," Trump tweeted on Nov. 15, 2015, of the UFC 193 main event the night before in Melbourne, Australia.

"[Rousey] was soundly beaten - not a nice person!" Trump added about Rousey, who had previously announced her support for Democratic hopeful Sen. Bernie Sanders (I-Vt.).

"It's easy for someone who isn't in there to comment," McGregor said of Trump's comments at the time. "It's different when you're in there - the emotions are high."

"I don't give a f--- about Donald Trump," McGregor added.

McGregor defeated Donald "Cowboy" Cerrone in a UFC 246 welterweight mixed martial arts bout on Saturday in just 40 seconds. It was McGregor's first win in the UFC in more than 3 years, as the star had taken a hiatus to pursue other ventures.

Trump has also received the backing of UFC President Dana White, who spoke at the 2016 Republican National Convention.