If he gets a hand cut off, he'll have to walk around without a hand (until he regains healing) but he will not die from it! Let me take back that comment about semi-immortality being something that needs to be regained, since that's part of his biology like the Houseki no Kuni gems: if he gets injured and lose body parts, he will not die because he is unable to be killed by destroying his core. But regeneration and healing will be considered powers and need to be regained. Does that make more sense?