Bush lawyer here.
If he goes against medical advice he only has himself to blame if he gets injured. If I was the surgeon I'd be getting him to sign an against medical advice form.
As it is an injury - as I see it, this is how it goes.
1) There has to be an assessment, followed by a diagnosis.
2) From that diagnosis, a treatment plan is set in place. The plan covers the treatment and recovery period, and activities that are allowed or not allowed during that time. The treating doctor has a legal right, and obligation, to authorise the prevention of activity that will jeapoardise the recovery process. It is not enforceable by law, but will rule any compensation null and void, and exposes anyone who contravened that authority to liability.
In other words, Doc makes an assessment, and a judgement on wether he can or cannot do stuff. He cannot say "Normally you would have to ly flat on your back for 3 months, but since you are Arthur Neddy Smith you can go and play golf tomorrow if you want".
It's an interesting issue though. When Johns played with the punctured lung he was at very real risk of severe complications and death. Likewise, John Sattler.
Joey got a medical clearance. It was the manly doctor, Nathan Gibbs, who made all the fuss. As for Sattler, there was no risk of complications, except him losing teeth, but that was normal with Souffs anyway.
If this did occur are the NRL liable?
No. They act on medical advice.
If he never played again could he have sued the NRL for loss of earnings?
Given that he is retiring and at worst he will miss one game, no.
Does the NRL have legal coverage to ensure that they are covered in a case where the player goes aginst medical advice?
No such coverage exists. But they are not liable for someone being a d*ckhead and ignoring their doctor.
If The Doctor says he can play, when we is unfit, the doctor is liable
If the doctor says he cannot play, and he lies to the club and said that he can, he is liable
If the doctor says he cannot play, but the club in any way co-erces him into making the decision to play, the club is liable.