Usyk eased to a unanimous decision win against Joshua in London in September, taking the WBA, WBO, IBF and IBO heavyweight titles from the Briton in the process.
The pair look set to face off again in 2022, with Joshua having triggered a rematch clause to fight the undefeated Ukrainian once more – likely in spring.
But “AJ” must be wary of a similar result, according to boxing icon Mayweather.
“That was not an upset,” he told reporters, via the Mirror.
“It was just that Anthony Joshua was on TV, everybody seen him. Usyk was a hell of a fighter, a gold medalist if I’m not mistaken, but he was behind the scenes.
“[They’re] two guys with crazy amateur backgrounds, and one guy is not being seen and another guy is being seen, then we call it an upset.
“No, it’s just that he was working behind the scenes and another guy was working in front of everybody.
“Finally, they had to meet up and Anthony Joshua came up short.”
Usyk’s professional record stands at 19-0 with 13 KOs, while Joshua is 24-2 with 22 KOs.
Joshua’s only other professional defeat came against Andy Ruiz Jr in 2019, with the Mexican-American stopping the Briton at Madison Square Garden in New York City.