Calvin Johnson is one of the Detroit Lions biggest superstars and he may be on his way out if he doesn't take a pay cut. Megatron is scheduled to count $24.008 million against the 2016 salary cap.
This basically means the Lions will be looking for a way to either re-structure his deal (moving guaranteed money into his later years 2017-2018), trade him (unlikely as they would not find a team willing to take that kind of contract), or simply cut him which would create around $12 million in dead money. The Lions are currently retooling their franchise (not quite rebuilding).
They are looking for a new GM and it is likely whoever that person is has a heart attack at the sight of Calvin Johnson's cap number. If Johnson became a free agent he would have his choice of teams to go to and would likely be picked up in an instant. At 31 Johnson is no longer the Megatron of old however he still can be an imposing force on the offensive side of the ball. 2016 Super Bowl contenders could be looking to add Johnson in hopes to increase their chances of getting into the post season.
Johnson recently said that it would be "lovely" to finish his career in Detroit however some unpleasant decisions for both sides still have to be made.