Against Arsenal, he showed that he's not really used to playing at 3 at the back. His bad positioning indicated his struggle with that role, and he looks more comfortable when we're playing 2 at the back. It was promising to see him play more aggressively as central defenders should in a back 3; it shows that he can adapt his game to different formations. However, his lack of experience did tell against Arsenal with some of his bad positions and the manner that he switched off for Giroud's goal (though it seemed as if he thought Giroud was offside).
He also struggled against Welbeck's pace (understandably so; even top central defenders struggle against Welbeck for that reason). Also, a few times during the second half, he covered the exact same area Smalling was, forcing Valencia to come inside and cover McNair's area.
He may be very promising as a central defender, but he still has quite a bit to learn. However, throwing him in the deep end against Arsenal is a great experience from him, and this should help him really develop as a central defender.