The UFC superstar was seen in a short film for a documentary, which is set to air later this year, on Friday, with a message for the fans.
“If you’ve ever had a bad day, there’s nothing worse than watching the UFC,” McGregor said in the clip.
“But this is not that day.
This is a day to celebrate.
A day to remember.
And I’ve had many bad days, but the biggest bad day was when I got injured.
That was the biggest.
But I’ve never been injured like that, and I’ve been through some great times in my career.
It’s never the same.
So I want you to know that.”
McGregor and Diaz have already fought each other twice, with the two of them winning in the first round.
They also faced each other in the main event of UFC 173 in March.
The two men will meet again on Saturday night, with McGregor and Diaz expected to take on UFC featherweight champion Jose Aldo.
McGregor has won three straight and Diaz three of his last four fights.
McDonald, a UFC Hall of Famer, will return to the Octagon for the first time since January 2019, when he was removed for a failed drug test.
He will be on a 12-fight win streak after winning a split decision over Anthony Pettis at UFC 183 in April.
“You just got to believe in yourself,” McGregor told reporters at the UFC 173 press conference.
“It’s going to be an amazing fight.
And this is a fight that I can’t wait to get out there and compete in.”