Some posters have mentioned this already, and I'm going to try my best not to be an echo so I'll put it like this.
What the Celtics want Amir Johnson to do right now is what I expect the Celtics to want from Jordan Mickey in the future with the only difference being that they expect Mickey to be a better shooter than Amir.
The highest ceiling I have for Mickey is being the prototypical small-ball 5. A rim protector who can run the floor and provide spacing. I see him having a great future here as a Celtic. I don't think he'll be a star, but I think he'll be a glue guy.