I think it largely depends upon what Denver, Philly, Boston and Phoenix do with their multiple 1st round picks. As it stands now, he is a long shot to be a first round pick, because several late first rounders will be used for draft and stash - there just aren't enough roster spots for all of those teams to use all of their picks on college players. The more of those picks that end up getting traded, the better chance he has of being selected in round 1.
Personally, I think he is a mid-second round talent, but all it takes is for one team to like you and you are a first rounder. I'd rather the Celtics not take him, regardless of where. He is a garbage man in the NBA and nothing more. Mickey does everything Brice can do and then some. Jordan's greatest strength is this team's biggest weakness (rim protection), and he still can't get minutes.
I see Johnson having a career similar to a lot of skinny, athletic post players. He'll rot at the end of someone's bench; he'll have a dunk or two and block a shot during garbage time and fans will say he needs to play more. Opposing fans will say "we need to trade for him because of what he did against our D-Leaguers". He will be a throw-in in two or three trades (with the new team's fans having huge hopes for him each time). In 10 years he will have had a long career hanging around the league with no accomplishments to his name.