I can see both sides of this.
- The argument for Bello as a sixth starter: it is hard/impossible to sign good, uninjured starting pitchers to roles without guaranteed major league starting opportunities. So even if Bello is, say, our third best starting pitcher, he should start in AAA to help us build "deep depth."
- This was basically the argument for the Paxton signing, FWIW. In order to add pitchers from outside the organization who don't need an Opening Day 26-man roster spot, something has to be wrong with them or they're not available for the role. An injured pitcher with high upside and a decent chance to return in the second half is one way of adding high-end SP depth.
- The risk is that you end up with some mediocre veteran throwing badly in Boston while Bello's development fails to progress in AAA.
- The argument for Bello to start in the rotation: we actually have a pitching prospect with ace upside, so we should prioritize his development, especially in a world where Kutter Crawford and Thaddeus Ward exist — i.e. where we have some other SP depth with upside. If he's able to realize even most of his potential, it's huge for the organization. And Bello doesn't have much more to prove in the minors.
- The risk here is that, well, our incumbent pitchers are not exactly the picture of health. Who knows what's going on with Sale and Whitlock, but no one would be surprised if they are injured at some point next year, and you don't want to put yourself in a position where Winckowski is throwing 90 innings for the big club.
- But... so much has to go wrong to end up in that scenario again that it doesn't really matter what you do.
I guess my view is that it depends on who we're talking about acquiring. If the idea is to sign or trade for legitimate ace types and really go for it in a high-risk, high-reward scenario, then sure, stash Bello and Crawford at SP6 and SP7 behind (let's say) deGrom, Sale, Paxton, Pivetta, and Whitlock and roll the dice — talk about a roster that could finish first or fifth. But if we're talking about signing Sean Manaea to be our SP4, I'd rather see what Bello could do from Opening Day.