Boxing legend Mayweather goes up against UFC superstar McGregor at the T-Mobile Arena in Las Vegas on August 26.
The pair have been involved in an ugly war-of-words in the build-up to the bout on their global press conference tour.
But Mayweather, 40, has sensationally revealed he believes 29-year-old McGregor holds the advantage despite having to learn boxing before the fight.
“He's a lot younger,” Floyd Mayweather said.
“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.
"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 cited his fight with Andre Berto in September 2015 as an example of how he has declined as he got older.
"I'm not the same fighter I was two years ago,” Mayweather added. “I’m not the same fighter I was five years ago. I lost a step.
“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.
"I used to have a 90 percent knockout ratio. Its obvious I slipped somewhere. Something has taken a toll on my career.”
But Mayweather insists he will defy many pre-fight predictions to defend against McGregor and plans to attack the Irishman.
"This can't be a defensive fight,” the American told ESPN. “I have to go to him.
"I owe the public because of the [Manny] Pacquiao fight. They weren't pleased with that. They're gonna be pleased with this fight here."