If they weigh in day of as they should, then no fight because Diaz will be at Welterweight. That's why you cant match up guys from 38yrs ago with the last 23 yrs or so.
Diaz didn't gain much weight. I remember when HBO used to weigh him. He was only like 140-142 in the ring. Mancini was probably around the same weight. Guys easily put on 5-6 pounds with a same-day weigh-in. You're on drugs if you think Juan Diaz is a welterweight in ANY era