On September 14, 2013, Floyd Mayweather defeated a young Saul Canelo Alvarez by majority decision due to the mentally deficient C.J. Ross. The fight wash a complete wash with Mayweather dominating the center of the ring and countering Canelo's lunges with his trademark pull counter straight right. It is important to note that this fight was for the WBC, Ring and WBA light middleweight title. The light middleweight class is 154 lbs, but the bout was 152 lb catchweight to accommodate Mayweather's naturally smaller frame and possibly dehydrate a bigger Canelo.
Today's Canelo is the superstar of boxing and likely would not have to meet additional demands from another fighter. The fight would take place at middleweight, where Mayweather has never fought. Mayweather would likely weigh in at the light middleweight limit of 154, just as he did for his last pro bout against Conor McGregor. Mayweather has shown in his recent exhibition matches that he still possesses a masterful skill set, despite displaying it against YouTubers, bodyguards, and kickboxers.
Canelo displayed the some weaknesses from the Mayweather fight in fights with Dimitry Bivol and Caleb Plant. When not the aggressor, Canelo is there to be hot. Fighters can bank rounds when advancing because Canelo is pressure fighter who counters. He draws shots by making the ring very small with footwork. In order to do this, he has to be either faster than his opponent, or more able to dictate pace and range. Against Mayweather, he will have neither advantage. Mayweather moves, creates angles, and will use an illegal elbow to the throat to push off an advancing opponent.
In a second fight, the winner is: shenanigans. I see the second act the same as the first but with an even more screwy finish. The result will read a draw. It protects Canelo and Mayweather from a loss. Canelo is the cash cow now in boxing and losing to a retired fighter will destroy promotions and future mega fights. A draw touts him as the one Mayweather couldn't beat. Canelo was fairly judged in losing the Bivol fight because their is money to be made in a rematch with Bivol and in Bivol facing Artur Beterbiev in a four belt unification at light heavyweight.