Floyd Mayweather explains how a lot of the measurable comparisons between he and Conor McGregor favor McGregor. (1:35)

Floyd Mayweather spent much of his promotional tour last month mocking Conor McGregor's skills and confidently predicting he would dominate their Aug. 26 boxing match, but he struck a more deferential tone Tuesday, acknowledging that he has "lost a step" and that McGregor, at least on paper, has the edge over him.

In an interview with ESPN's Stephen A. Smith, Mayweather, 40, pointed to McGregor's advantages in size and age as reasons why the UFC fighter poses a legitimate challenge.

"He's a lot younger. When you look at myself and Conor McGregor on paper, he's taller, has a longer reach, he's a bigger man from top to bottom. He's a lot younger, so youth is on his side," Mayweather said of McGregor, who is 29. "And I've been off a couple of years. And I'm in my 40s. So, if you look at everything on paper, it leans toward Conor McGregor."

Mayweather, the former pound-for-pound boxing king and five-division world champion, is coming out of retirement for his clash with McGregor, a bout for which he is widely considered a heavy favorite.

He has not fought since his one-sided unanimous decision over Andre Berto in September 2015, which improved his record to 49-0. McGregor, the UFC lightweight champion, will be making his professional boxing debut against Mayweather.

Asked by Smith why he wasn't brashly proclaiming McGregor has no chance in their fight, Mayweather said simply, "I'm older.

"I'm not the same fighter I was two years ago. I'm not the same fighter I was five years ago. I lost a step," he said. "A fighter like Andre Berto isn't even supposed to go the distance with Floyd Mayweather, but remember, I was 38. It's obvious I'm slipping a little bit to even let a fighter like that go the distance with me.

"I'm not what I used to be."

One major difference Mayweather pointed to as an older fighter was his knockout power -- or lack thereof. His last knockout came in 2011 against Victor Ortiz, and his past seven fights have gone the distance.

"I used to have a 90 percent knockout ratio," Mayweather said of his punching power early in his career. "It's obvious I slipped somewhere. Something has taken a toll on my career."

Mayweather, however, emphasized that he believes an "older" Mayweather still reigns supreme against McGregor, telling Smith, "I didn't say I couldn't fight. I just said I'm not the same Floyd Mayweather I once was."

As for his streak of fights without a knockout, Mayweather, known for his defensive wizardry in the ring, said he believes he owes it to boxing fans to be more aggressive and take the fight to McGregor. Mayweather specifically referred to his decision victory over Manny Pacquiao in their May 2015 megafight, which didn't have the action or drama that many hoped for.

"This can't be a defensive fight. I have to go to him," Mayweather said of McGregor. "I owe the public because of the Pacquiao fight. They weren't pleased with that. They're gonna be pleased with this fight here."