I agree, I think it's FB command. Also FB confidence. He doesn't seem to have any of it. Lester had the same issue, so it's not insurmountable.
Also I don't care much for his game plan so far in his starts. What I mean is, he doesn't save anything. He shows every batter everything he has the first time through. Making him look dominant. Then by the 2nd or 3rd time through the order everyone knows what hes got, sits on certain pitches and Tee's off on him. He needs to stick to FB/CU or FB/CB first time through. That way he's got something different to show them the next time. And then the third time it's a guessing game. That will help him get later into games. Pitch a little more to contact. If he does that. The Sox are in good shape.