A good (and somewhat popular) topic addressed by Chris Forsberg in his mailbag.
As for why the Celtics won't give Johnson a chance, I think there's a couple reasons. We all know Celtics coach Doc Rivers is reluctant to play rookies, particularly if he's not confident in their ability to grasp the playbook (and Johnson had that much-publicized moment in Toronto where he drew Rivers' ire for not being in the right spots). Overall, I think Rivers likes what he's seen in Johnson's development during his rookie campaign, but with such limited practice time, it's very difficult for a young player to develop that much-needed trust to earn floor time. Rivers also said something revealing a couple weeks back when he noted that Boston simply isn't sure "if JaJuan's a big." The insinuation there is that, while he's got size, he might be more of a wing player at the moment and the team needs to bulk up his frame before he's ready to go rumble down low in the NBA.
Personally I don't have a problem with this. If the kid isn't ready, he isn't ready. I know Doc is slow to come around on rookies but I'm fine with that considering this veteran roster and the goals of this team. Next year might be another matter entirely.