I don't think Morse should be included as a "make it or break it". He's already made it and has had a good career. Nothing left to prove, but yes his concussion history should be a concern and I would not blame him for retiring after next season in the best interest of his health and his future.
I would include "Rick" Bates though. He can prove this year that he's the potential center for the future. It would be great if he is!