That's fine -- I can understand the skepticism based upon the viewpoint that past performance is often an indicator of future performance.
If Sidibe is shot physically [a la Coleman], then I see your point.
But if he bounces back, then he has a lot more upside in the post than Marek does -- as a rebounder AND defender. I tend to believe that last year was a lost cause -- he was rehabbing right up until the preseason games and wasn't able to spend any time over the offseason working on his game. Then he got off to a poor start, which led JB to Booheim him, he got limited minutes, and lost confidence.
I don't know whether Sidibe can get healthy. But if he does, then his baseline isn't what we saw last year.
And if he can't, then it might be.