The American star retired from boxing in September 2015 after cruising to another points win as he beat Andre Berto.
His win over Berto stretched his record to 49-0 but Mayweather will return to the ring on August 26 as he prepares to do battle with UFC star McGregor.
The duo will meet at the T-Mobile Arena in Las Vegas and the veteran is the overwhelming favourite for the bout.
However, the 40-year-old has started the mind games by claiming his age and inactivity will play into McGregor's hands.
He told ESPN: "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. 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 Andre Berto go the distance with me. I'm not what I used to be.
"I used to have a 90 per cent knockout ratio. It's obvious I slipped somewhere. Something has taken a toll on my career.
"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 40's. So, if you look at everything on paper, it leans toward Conor McGregor."