To clarify, fans aren't upset because O'Brien might be fired.
The fans are upset, and many want him fired.
Last week I didn't think he was on shaky ground. But I do now.
Losing Watson wasn't his fault. But the feeling is that his "backup plan" at QB, Tom Savage, was really no plan to speak of.
And for being billed as a quarterback guru, O'Brien has been anything but. Their carousel at QB the past few years has been an embarrassment. 
What's worse, you have one of them, Case Keenum of the Vikings, go on not only to be serviceable, but solid.
Add it all up and O'Brien could go. But hardcare fans will tell you that GM Rick Smith deserves to fired more than O'Brien.