Out of all of our outfield prospects, Mitchell is the one I'm leery of the most and the one I'm most willing to part with. Yes he's awesome in the OF and possesses above average raw power, but he'll never tap into that raw power unless he completely changes his swing and the K rate for a guy who relies on his speed is alarming. Man, would I love to get Edward Cabrera from the Marlins for him...
That being said, I agree that you can pretty much pencil him into CF on Opening Day as he already has that MLB experience and didn't fall flat on his face (at least according to the surface level stats). Though, the Brewers do have an incentive to play Frelick right away as a top 100 prospect who could realistically be in contention for ROY....