Not likely..
I doubt they regret extending Rafaela, even though he is one of those with the defensive reputation, but unlike White, he at least plays a position in which it can have a greater impact. As his AAV is only about $6.25mill, it might not be such a major regret, and the Sox have swallowed much bigger contracts, like Sandoval, Hanley and Rusney Castillo. Let's say Rafaela has a ceiling of Byron Buxton - 4.6 to 4.9 bWAR. If he reaches that, he is a bargain. But his floor is probably closer to Kevin Pillar (~1.5 to 2.0 bWAR). In the case of the latter, he still might not be a horrible investment, unless he gets replaced as a starter. Of course, injuries might change this somewhat.
Bello's extension only carries a $9mill AAV. So even if he flounders, this type of money gets you a Wade Miley/Alex Wood type pitcher today. Miley is probably a bit underappreciated (and an insufferable ******* according to some) and if Bello can maintain that level of performance, the Sox will not likely regret it too much. Really unless he succumbs to injury problems, the Sox have a guy paid like a mid-tier SP who has potential to be much more. If he peaks at a mid-tier performance, he is not untradable under his current contract. I'd say his chances of not living up to his deal rest solely on his ability take the mound every 5 days...