According to Ian Browne of MLB.com, Red Sox president of baseball operations Dave Dombrowski said, “He’s still not an established Major League player […] He has to go out and show he can earn the job. And the way you go out and do it is to play day in, and day out. He has to go out and do it. Do we think he can do it? Yes. But he still has to perform and show us that he can.”
The problem was that Castillo was bitten by the injury bug early last season, to the point where even when he did play he was not playing to his potential.