He got lucky with one punch against Williams and was using his face to stop the kid's punches in those rounds. Good luck dropping weight and using your face to stop Pacman's punches.
That's as dumb as saying that Mayweather would use his face to stop Pacquiao's punches, well no sht, it goes both ways at that level of talent. Sergio blasted Williams in the first round, see 4:36 for the flurry that would have dropped most other fighters... Paul has a crazy chin though and of course he's going to land punches too, he was top 5 pound for pound in the world and that was only his 2nd loss ever. If you want to see some real exchanges, watch Martinez-Williams' first match, instant classic.
Pacman would most likely have to move up to around a 105~ catch weight to make the fight happen, meaning he gains 10 lbs and Sergio loses 10. Of course, the day of the fight it'd probably actually be more like 162 vs 145 haha.