Pain and injury are separate things. They play with pain all the time and some even with injuries. A medical staff will spot a faker really quick.
If I say that I have recurring headaches and dizziness, what is a medical staff going to do about it... prove that my headaches are faked? What if I say that have soreness in my knee, and I don't want to risk long-term damage with painkiller injections?
A medical staff can clear you as able to play, but they cannot evaluate your pain threshold. While many players do choose to play through pain, there is no clause in their contract that specifies a minimum amount of pain that the player must play through.
If a player
doesn't want to play, and chooses to claim that he is in too much pain to play safely, no team will force him on to the field. They can cut him. They can attempt to get back money that is
guaranteed only for injury. But fully guaranteed money is gone.