This is either physical (he's injured or ill) or mental. I think it's the latter. I know he's 20, but MLB pitchers and big parks shouldn't overwhelm him like this. He has been around this all his life, through his dad.
I have to admit I am a bit at a loss. Is the gap between AAA and MLB that huge? How can someone dominate at AAA and then all of a sudden be horrible at the MLB level? No one was expecting him to tear it up immediately off the gate, but at least hit .200 with some extra-base hits.
The Orioles drew the top two hottest teams in the NL (Pirates and Brewers) in the first two weeks in the season. When the NYY play them in the middle of the season, they will be cold as icicles. Just watch.