Last year, the only thing that kept Garrett from getting fired was his guaranteed contract. Not so much starting now. And JJ can make things very uncomfortable for Garrett to force him out.
Start with having him fire his brother. Take playcalling away from him. Not many cards left for Garrett. Either he accepts it and keeps his job (for the time being) or he resigns. Resigning would make Jerry Jones very happy. Don't hacve to pay Garrett anymore and get a better head coach, not someone who is prone to brain farts.
My guess is that this is Garrett's final year here, and there is a high likelihood that Garrett gets canned mid-season.