I think it is fair to have high expectations of Rosario. Any idiot can take a pitch, its not a skill. But players need to understand why its important to work deep counts, and it doesn't look like many of them do. Maybe coaches could do more in that regard.
The league bats 1.796 in 3-0 counts, and 1.366 in 3-1, .976 in 2-0. If I were Jake Mauer, I'd move the goalposts away from "drive the ball" to "get to those counts." The aggressiveness will take over from there, it doesn't need encouragement, and the walks will follow incidentally, as they should.
Pair that with an emphasis on 2-strike hitting in order to keep strikeouts in check.