I think he's got a lot to learn, but his strength and pace are huge assets - if he can improve the positional side of his game, having him at CB would allow us to play a much higher line when we want to dominate the ball against weaker sides. I wouldn't go as far as to say he's "vulnerable" in the air, but he isn't as good as Keane or Mina in that regard. Centre back has to be his position, because he isn't good enough on the ball to be a viable long-term prospect as a full back. Has a lot going for him, but you're absolutely right - he isn't anywhere near the finished article yet. Benitez will probably be a good manager for him though.