Unless wiser members say otherwise, I'd try to tease out some from the outer edges. Failing that, take a sharp knife and score vertically around the bottom third to half of the root ball, and the very bottom, cut through the root ball with each slice about a third of the way in/deep. I'd also score the sides of the hole or rough up the sides a bit, to encourage the roots to grow out into surrounding soil. It sounds so potbound now that a smooth-sided hole would only make the roots think they're still in a pot. Also, no amendments in the hole; just the native soil you dug out, otherwise the same "pot mentality" may still apply. In spring after it leafs out then work in some amendments of choice.