I've been under the assumption that there have been injury settlement negotiations going on behind the scenes. It feels like Tua and the Fish are playing a game of chicken. If Tua's is not allowed to play because of his injury, he gets paid. If he quits because of his injury, he doesn't. So far, neither side is flinching, and the NFL has declined to get involved (which is tantamount to siding with the Dolphins).
And still, Grier, and the Dolphins have done nothing to shore up their porous O-line, and give Tua some modicum of protection. Instead, they will continue with their get-the-ball-out-of-your-hands-as-fast-as-humaly-possible offensive scheme. Maybe they're hoping that opposing defenses will be easy on him, rather than take the risk of killing someone on the field?