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Cotto and Mayweather after their fight came to an end.
Mayweather has fought in the junior middleweight division only twice in his career, both times against high-profile opposition.
Why doesn't he return to that division? There's literally no one left who can beat Mayweather.
Mayweather defeated the biggest fighter of the division in Miguel Cotto (37-3, 30 KO) this past May.
Saul Alvarez (40-0-1, 29 KO) is a 22-year-old phenom, but his competition hasn't been anywhere near Mayweather's level.
Alvarez and his handlers plan to take baby steps to greatness, which is all fine and dandy. The only problem is that a 35-year-old Mayweather doesn't have years to wait until Alvarez is ready.
That would leave Sergio Martinez (49-2-2, 28 KO), a 37-year-old middleweight champion who used to fight at junior middleweight not too long ago.
Martinez has a big high-profile battle against the immensely bigger Julio Cesar Chavez Jr. (46-0-1, 32 KO) on September 15.
Even if he wins, Martinez won't be ready to fight Mayweather in December, let alone November. He'll need three months just to fully recover from what promises to be a brutal event, win or lose.
With all that said, Mayweather will likely never venture to the junior middleweight division ever again.