Monzon would handle the declined SRL that beat Hagler. Leonard would be competitive early on but Monzon would would come on and win a decision. I don't feel he would stop him over 12 but the stipulated 15 might get tough. I think the peak version of Leonard pre first retirement would give a fine showing but fall short by the final bell. Monzon had that stamina and would make the smaller man work hard to stay in it. Ray, a long range specialist himself, would likely wilt as the bout slid into the championship rounds due to the size disparity and ability of Monzon to keep the pressure on. I'd still back him to last the distance.
Monzon was very good maybe great but he didn’t have the competition Leonard and Hagler had. Monzon never had to fight a Mugabi Benitez or Hearns. His best opponents were Griffith and Napoles both older Welterweights. Briscoe was tough as nails but not on Ray or Haglers level. Benvenuti was fairly good but at the end of his career. The rest of his opponents I wouldn’t want to put money on against Roberto Duran as a middleweight.