As long as Mayweather would not fight Pacquiao, the Mayweather-Pacquaio who-is- the- best- of- em- all would just be become subjective. And we can't do anythin' if people try to compare how each of em (Shane and Manny) fares to Mayweather.
It's like a poser: WHAT WOULD HAPPENED IF MANNY GOT A SPLIT DECISION WITH SHANE? OR SHANE KO'D HIM?
We can not avoid people dissing Manny as an inferior boxer than Shane despite both of them did not duke with each other. That's their opinion. That's subjective.