Brook had been hoping to share a ring with the WBC, WBA and WBO champion in his planned farewell fight in Las Vegas on September 12.
The Sheffield star extended his unbeaten record to 35 straight wins, 24 by knockout, after stopping Frankie Gavin in six rounds in the second defence of his title last month.
However, neither Brook or domestic rival Amir Khan were in Mayweather???s thoughts when he named Andre Berto and Karim Mayfield as possible opponents for his 49th fight.
Berto, who holds the WBA ???regular??? title, has lost three of his past six fights while fellow American Mayfield was beaten twice last year and has never fought for a world title.
Brook???s promoter Eddie Hearn has responded by admitting a fight with Mayweather is looking increasingly unlikely and is instead making other plans for the 29-year-old.
Hearn told ESPN: “We would love to make the Mayweather fight for Kell on September 12 and we are in the hat because Kell’s world champion and if Mayweather wants to become undisputed champion with all the belts he has to take that fight.
“I don’t think we will get it and others have been mentioned. Kell Brook is more creditable than the names that have been mentioned and like I said Mayweather could have the chance of unifying the division, but I don’t think it will happen.
“We are going for Kell’s next defence to be at the O2 Arena on another huge show there and it’s looking like it will be against Brandon Rios.”