Ryan Tannehill will be the quarterback in 2018, or at least he has a 90% chance if not better of being that QB. The Dolphins could always trade or release him and the $4.6 million in dead cap money is very edible. Miami could also opt to restructure his contract.
The problem Miami faces however is the unknown condition of his knee. Will it tear again? Will it mentally or physically keep him from running the football? A crucial part of his game as a quarterback. The Dolphins will have to weigh this as they look towards the 2018 season.