I don't know if theres any welterweight @ the moment that could figure out the Paul Williams puzzle. He just creates too many stylistic matchup problems.
Its one thing that he's got about a 6 inch height/reach advantage over just about everyone else in the division. ( Cotto is only 5'7 ) But his punch output is insane...and if you're not throwing him, chances are you're not outlanding him either. You can't beat the man from outside, a la Floyd Mayweather, he's too damn tall, and knows how to use his jab/height...he's got a really solid inside game, and good enough chin to take shots from one of the, if not the hardest hitter in the division. Plus, he's a really slick southpaw, that has good footwork, and suprisingly good balance for someone his height. Might not have loads of power, but he's going to outpoint you, simply by his volume of punches...and its also hard to throw punches when you're busy blocking them.
I was really impressed after re-watching Saturday's fight. This kid looks good. The only thing that was iffy to me about him, before the fight, was his chin, and it definitely didn't look like glass on Saturday. He took some clean shots, and kept pressing.