Two thoughts - First, since the school fired Miles, I could understand players losing focus and beginning to think about themselves. I am sure that many feel that the school gave up on them when they gave up on Miles. Second, if Fournette (or any player) half-steps in SEC games, they are risking injury. For that reason, he should try to be himself on the field or get off the field altogether.
As someone, who has a lifetime of weird health problems, I can not fault any athlete, for waiting until they are fully healed, before getting back on the field. Some others (read: Tony Romo) need to learn how to ratchet down the competitive drive, and realize they are probably not going to be able to be vertical, when they hit 60. (Or, in his case, 50.)
Depending on how long it takes that nagging injury, to heal, and what the team's situation will be, once he is fully healed, should determine whether it is quitting, or not.
Yes, I know some of our players have gone through an entire season, really banged up. They are gutty performers, and have earned a lot of well-deserved respect. I only hope when they reach my age that they won't regret what they did. Only they will be able to answer that.
(Not that it matters, but tearing up my right shoulder, over 50 years ago....................it hurts, as I type this. Frequently makes handwriting dang near impossible. Adhesions, scar tissue, etc. And, no, it was not worth it! I still flunked gym.)