You can probably leave your employer and work for someone else whenever you want though.
Contracts pretty much universally work against the players in the NFL. They get drafted, they get traded, and then they don't really get much protection with contracts despite facing significant limitations because of them. NBA and MLB players are a lot better position. You get some funny situations like ARod getting $20 million/year for being one of the worst players in baseball, but I suppose that's the reward for being an all-time great earlier in his life.