The 53-year-old actor said he had an urgent dental appointment that made his “mouth feel like fire" a couple of days before the special, which aired in May 2021 and was hosted by James Corden.
“It sounded like my voice was up ... I couldn't not show up," he told Diane Sawyer in a new interview on ABC. “So, what I chose to do was just go and do the best that I could."
Perry was joined on the show, which was a sit-down chat looking back at the popular sitcom, by his former castmates Jennifer Aniston, Matt Le Blanc, Lisa Kudrow, David Schwimmer and Courtney Cox. The stars were also joined by a string of celebrity guests.
In the ABC chat, he became emotional when he reflected on seeing himself play Chandler Bing in season three of Friends because of his “painfully thin" frame.
“It's very hard to watch that, because in this weird way I feel sorry for that guy, because that's a guy that's out of control," said the star. “I didn't know what was going on with me. I weighed 155lb [70kg] on my way to 128lb [58kg].
“I feel too sorry for that guy, he's going through too much – and it's me, and I remember that, and I didn't understand what was going on.
“But again, I'm sorry, and I'm so grateful to not be that anymore ... but it's hard to, it's hard to watch that."
Perry is currently promoting his memoir Friends, Lovers, and the Big Terrible Thing, which touches on his struggles with alcohol and drug addiction.
In the candid memoir, he also reveals that he was the one who ended his romance with Julia Roberts and makes his negative feelings towards John Wick star Keanu Reeves clear.
Friends, Lovers, and the Big Terrible Thing will go on sale on November 1.