There are definite signs that he may have lost the locker room, or at least a significant portion of it.
If that is the case, you really do have to let him go. You don't recover from that.
Just because players are frustrated, does not necessarily mean Rivera has lost the locker room. Plenty of teams have rebounded from frustrating seasons.
IMO, this whole season comes down to 2 points. 1. Shula and his style of offense. 2. The talent this offense has/doesn't have.
Upgrade the OC and offensive talent, and maybe we can score some more points. To me, that would add some much needed enthusiasm to the offense, as well as taking a poo ton of pressure off the defense.
In short. Just maybe Rivera and Gman understand that Shula and his offense, along with the dearth of offensive talent in general, have more to do with our underwhelming season than anything else.
We as fans can react anyway we wish. Calling for heads to roll is our right. Unfortunately, the FO can't think that way. Giving Rivera the tools he needs to succeed could very well fix our ails.
Rivera may be conservative by nature. And he may never go for a wide open, pass happy offense. But he may be fine with an OC that has the imagination to give his players plays that work to their advantage.
tl/dr version. Get rid of Shula. Upgrade the offensive talent. Then see what Rivera can do.