The issue with Harrison is that he has, on the face of it, all the qualities to be a top quality footballer. He is tall, strong, a decent header of the ball and an absolute unit for his size. Unfortunately, I think he lacks the brain of a top-level footballer at the moment. He seems to need some direction, prompting from others around him and could benefit greatly from some game time and playing alongside a centre-half partner (or coach) who teaches him to read the game.
He seems a bit slow on the uptake, when reading the game and that leaves you out of position. I have hope for him though. He really needs to go out on loan. Somewhere like Northampton would be ideal; give him a bit of independence, and learn from established pros like Ryan Cresswell and Evan Horwood. Wilder could be good for him too.