Players are coming in to the NBA so much younger these days it is really tough. I always think of Chauncy Billups and to some degree Joe Johnson. I felt with both of these players, you could see enough of the talent but the Celtics (and in Billups case some other teams) moved on. Other players, like Fab Melo, you just know right away he was a bust. He just didn't have the reactions to keep up with the game. Then there is Avery Bradley. I believe with him, it you tried, you could see the talent but you could also see with him that he was emotionally too young for the NBA that first year.
I did not see many (if any) Sac games last year so I won't even pretend to know anything about McLemore but a bad season on a bad team alone is not reason to give up on him. If you have watched a bunch of the games and formed that opinion, that would be valid but it would be risky in my opinion to read some online print and then immediately write him off. (You certainly couldn't come away negative after watching the video clips where he never missed a shot).