Is Chris Johnson someone who would do enough of what Doc wants on defense (more than JJJ, at least) so that JuJuan Johnson would have a clear illustration using a similar player of what he has to do to earn playing time?
I don't think so, from what I remember and have seen, he's a horrid interior defender. Good shot-blocker with his length, but terrible at actual defense.
I'd be more concerned about his pick and roll defense and his ability to learn defensive rotations than I would be about man-to-man or post defense. Or, rather, I think that Doc would be more concerned about that and a willingness to buy into a team defensive scheme that can cover for some individual shortcomings.
As for rebounding:
JuJuan Johnson 8.7 ORB%, 13.1 DRB%, 11.0 TRB%
Chris Johnson (this season) 9.1 ORB%, 12.6 DRB%, 11.2 TRB%
Chris Johnson (last season) 10.3 ORB%, 18.2 DRB%, 14.2 TRB%