Juan Diaz was coming off a loss to Campbell. David Diaz was also penciled in, even before the Marquez rematch.
at that time, Pac should've fought Nate Campbell. Pac was only using the Diaz fight as a stay busy fight for the Oscar fight. It was only 3 months after his war with Marquez where he got cut. Pac probably wasn't even at 100% in the Diaz fight because he sure as hell didn't take much time to recuperate after his March fight and also it was his first fight at 135.
He probably should have fought Nate Campbell rather than Juan Diaz. Juan would have certainly been a better opponent than David atsch None the less he would have unified the titles at Lightweight if he wanted to.