He's playing in a position where experience is incredibly important to having dominant and consistent performances. I think for a 21 year old CB in the top flight, he's doing a very good job and regardless of any mistakes - he's playing well beyond how I'd expect a pretty young player in a defense for a top side. Young defenders have it pretty rough at a top club, because mistakes are under a microscope - while young attacking players making mistakes don't make as many mistakes that are as costly (in terms of the likelihood of giving up a goal).
I think he's going to end up a very good defender, barring any horrible injury problems throwing his career off. But with young defenders, mistakes - and sometimes costly ones - are going to happen. It's part of their development, because they need to learn from those mistakes and how to prevent them and not let the criticism and pressure from making mistakes get to them.
A little patience and realism with young players, especially young defenders, is important if you actually want to see Chelsea develop players that make it into the first team. Otherwise you'll see shite like getting rid of players with promise to really kick on leave, go somewhere else and shine, and then be a reminder of what could have been.