Johnson is a SG and best playing there. He doesn't play physically enough to contain the top 10-12 SFs in the league and has never been able to be the leader your team desperately needs him to be.
One thing that confounds me about this type of draft is the way people draft players who play nearly there entire careers at a position and just move them to another and claim they will be just as effective in that position. There's a reason they play the position they do and there is a reason the have success at that position. Expecting the same results while having to play offense and defense against an entirely different group of players with different skill sets is just not logical to me.
The center position I'll agree to disagree about, but you and IP both seem to feel that Johnson can't play SF and I don't really get that. This year he hasn't been there as much because there's no Jamal Crawford, but last year he spent 1/3 of his minutes at the 3, and defended it excellently (11.6 PER for the opponent at the 2, 12.9 at the 3 - both phenomenal). He was also better offensively at the 3, with a 17.8 PER compared to 15.9. The year before, he played ~25% of his minutes at the 3 and defended it better than he defended the 2, with equal offensive production of over a 20 PER.
It's not like I'm taking a guy who has never played a certain position and arguing that he's going to excel there. I'm taking a guy who historically has played significant minutes at the 3, produced arguably better there than he did at the 2, has an ideal physical makeup for the 3 and has guarded both positions very well in the past. He's obviously spent most of his career as a SG, but it's not like he hasn't proven himself to be equal, if not better, as a SF. I'll accept that it's going to be taxing to play with more physical players most of the time, but the guy is a solid 240 pounds and the strength is offset by the smaller amount of quickness.
Just my thoughts, but luckily in this game we are entitled to our own opinions
