LAS VEGAS (Reuters) – Floyd Mayweather rounded on his detractors after beating Mexico’s Juan Manuel Marquez with a unanimous points decision on his return to boxing on Saturday. “I’m never going to win,” Mayweather, 32, told a post-fight news conference after dominating his first bout since coming out of a 21-month retirement. The American has been criticized in the past for avoiding some dangerous opponents and he was loudly booed by a majority of the 13,000-strong crowd as he made his way to the ring in the wake of the hugely popular Marquez. Although Marquez was a world champion at three different weights, there was criticism of Mayweather’s selection of a former featherweight champion, who had most recently fought at lightweight, as his opponent for a welterweight bout. That criticism intensified when Mayweather failed to make the contracted weight of 144 pounds on Friday, instead tipping the scales at 146 pounds, four pounds heavier than Marquez. “To be the best, you got to beat the best in your era,” said Mayweather, who was back in the ring for the first time since his 10th round stoppage of Britain’s Ricky Hatton in December 2007. “It’s not about weight classes.