The choice of who is sent to the pen or AAA should be based on a few things, including how each pitcher has looked in his last 2-4 starts, options remaining (although I hope and don't think this doesn't lead to a Bello demotion), as well as who might be a better fit for the pen, or even who's psyche is better suited to handle a demotion to the pen.
It's obvious Pivetta is pitching the worst of the 5, but he has also been the most durable and consistent starter on the Sox for over 2 years. I doubt that is enough to tip the balance to picking someone else.
By the time Paxton pitches and the next 4 starts afterwards, we will have a chance to see one more start from the "candidates," or if Paxton gets drilled, maybe we let all 5 start one more time, before Paxton's next start.
Usually, one might think it's a good choice to have 6 guys to choose from, but in our case, it's trying to choose the lesser of 6 evils.