Perry gave tiredness as a reason for at least one lackluster debate performance in the past, but the interview was the first time he admitted that the back surgery may have been the root cause of widely panned debate stumbles.

“Looking back and trying 'woulda, coulda, shoulda' is an interesting question to ask but the facts are the facts,” Perry said. “I was pretty fatigued.  But no excuses, it was there, it's what it is and, look, if anybody's looking for a perfect candidate I'm not it.”

Perry again defended himself as not the smoothest debater, and said it comes naturally to him to make fun of himself for missteps. Perry appeared on "The Late Show with David Letterman" to deliver a self-mocking "Top Ten" list following a debate in which he forgot one of the three federal departments he proposed to eliminate as president.

“Anybody that stands up for public service is going to have some things happen to them and the media is going to report what they report and, again, I'm not perfect and I forget things, I misstate some things but I think Americans are looking for somebody that will admit when they're wrong, admit when they make a mistake and don't even mind poking some fun at themselves,” he said. “So, yeah, it comes pretty natural for me to go, yeah, I stepped in it and press on and stay focused on the work at hand.”

Perry has refocused his campaign with an emphasis on retail politics in Iowa and is kicking off a two-week bus tour this week to visit 42 Iowa towns. He earned 8 percent of the vote in a University of Iowa poll of likely Republican caucus-goers on Monday, while an American Research Group poll also released on Monday had him at 13 percent.

Perry entered the race as a frontrunner in August—he told the Iowa TV station he made the decision to run in late June of this year—but quickly dropped dramatically in the polls. The three-time Texas governor said he has come up from behind in previous elections and dismissed the idea he ever considered the nomination an easy win.

“Life is not supposed to be easy. As someone said, if you're not getting beat up every now and then you're not playing at a high enough level,” he said. “I love my country, I think I've got some ideas about the economy, how to get this country back working again. … I think Americans are looking for in particular is someone who has got a set of principles, got a set of values, who has been consistent in their life and if nothing else I have been consistent about those values and particularly about creating jobs in Texas over the last decade.”