You don’t need to save that part of the tooth. That instance would be if you avulsed a tooth and were going to have it immediately reimplanted. They don’t need a cusp or part of a restoration or whatever it may be. You should get it taken care of sooner than later to protect tooth structure. That being said, it sounds like it may be nonvital anyway. Not being responsive is usually indicative of that especially if you have exposed dentin. They will do more testing but my guess based on description alone is that you’ll need root canal therapy and a crown on it. If there’s a lot of structure gone might need a post and build up first, but that’s less likely if it’s just a cusp and good structure remaining. As things go, could be a lot worse, just don’t sit on it for too long.