I go with Rosario big. Rosario at light weight before Chavez looked like a ATG in the making , he had devastating power in both hands . Mayweather was well skilled, A big right hand. But chin was below average...... At best. I see Mayweather winning early rds, But Rosario would eventually test his chin. A test Maywather would fail.
While Rosario was a little fragile himself, and tended to come in out of shape his combination of power and precision is a serious problem for Roger. Most likely scenario is Rosario melting Roger like candle wax.
Who ever landed first and most likely that would be Roger. But if Rosario so much as Touched Rogers chin with that right hand Roger is done.
I wouldn't have the most confidence in Rogers chin in this one. Especially against a prime and focused Rosario. He was extremely dangerous.
I think Rosario would knock him out easily. Don't know why I see it. Roger was knocked out by guys who were not that great.
Roger Mayweather was ko in 2 by Ray Lovato , Julio Casear Chavez and Rocky Lockridge in one. Rosario won the LW title with ko 2 of Livingstone Bramble. Rosario by ko in 2.