Mayweather picks him apart, Valero was a no defence, all-aggression southpaw who relied on his power. Mayweather moves to his left, establishes lead-foot dominance and picks him apart with straight-rights. I'd say Ortiz is the closest comparison to Valero that Mayweather fought.
Mayweather. Valero has nothing but southpaw awkwardness and power. If those were enough to beat Floyd, even with the one arm he had Pacquiao would have dissected Mayweather.
Valero. Mayweather didn't like boxing southpaws and would have been vulnerable to Valero's straights to the body.