A lot of things would factor into this decision, as mentioned above. I think the OL will start out better than it did last year. Green is gettin old and has had a concussion, but he has a history of being very durable. KC's OL was not so great last year either having lost their 2 best players, plus Herm's offense did not seem to be too well suited to Green. Cam's offense is similar to Vermiel's, and he should be very comfortable running it as he had 2 years under Cam while with the Redskins. The concussion was bad, but testing has shown no lasting effects. I also read a report that said due to the healing time he has had and other factors, Green really isn't much more likely to get another concussion than any other QB out there. His body may have slowed a bit, but there is no reason to believe his mind has. What the Dolphins need is a QB who can make quick and smart decisions. Green can do that.
As for who would replace him, it depends on why he needs replacing. Early season injury, it's probably Lemon. Late season, poor play, no playoff hope, I'd bet on Beck.