On one side, the kid looks at least as ready as Pedroia did in '06, has looked great in his limited chances in the bigs, and has shown every tool you could expect of him in AA/AAA. I think he could handle himself over a full season in the bigs, and just as importantly, I think Theo thinks he can handle himself in the bigs. He'd struggle and slump a bit but he's going to need to learn how to get through that anyway, so if you think he can get the job done in the outfield, there's no reason not to debut him next year.
On the other hand, there IS an argument to be made for going into FA looking for Werth or Kalish, established veterans that can help the team as we try to get into the playoffs next year. Since it hasn't been less sure we'd even make the playoffs in a long time, going the safe route and bringing in a vet could save our season, and rookie growing pains could sink an otherwise playoff team if Kalish has a hiccup -- and by the time you'd know one way or the other, if you don't sign Werth or Crawford in the offseason, it would cost roster talent or high end prospectws to fix.
Not sure where I come down, but I do like Kalish and believe in his potential longterm, so if Red Sox nation can stomach the prospect of a second bridge year if things go sour I think it's appropriate to take the chance on the kid -- who knows, with Ellsbury and Kalish burning up the basepaths at the top of the order this team could go forther than an old strikeout hitter like Werth or a very very talented but occasionally inconsistent guy like Crawford, could take us.
And of course, there's a third option. Kalish is a potential 5 tool OF prospect who looks close to big league ready, other teams like those for some reason, and we have some holes to fill...