The only way you get value out of guys like Bello and Whitlock is by playing them, not stashing them in the minors to answer what ifs. I think it is likely that we see the starting rotation be, Sale, Paxton, Pivetta, Whitlock, and Bello. Hill will probably come back to add depth and there will be probably a couple more Hill/Wacha signings to provide additional depth. I also don't think we are going to see a tremendous amount of movement from the Seabold/WInchowski/Crawford group since they will be needed for depth. In addition, Mata and Walters should be able to provide additional depth. I guess I can see another mid rotation starter signed or traded for. Whitlock only pitched seventy eight innings last year, so it might make sense to limit his innings by starting him in the pen and then having him step in when someone goes down. At the end of the day though, when you have players like Bello and Whitlock who have shown they could be potentially mid to top of the rotation potential, you don't get cute by sticking them in the minors for depth. You play them so they can live up to their potential.While Bello doesn’t have the same injury related uncertainty as the others (knock on wood), he does have performance related uncertainty simply because he doesn’t have the track record to be able to confidently predict anything. It would not be terribly surprising if he struggles next year and needs to go back to AAA for a some more seasoning.
Think about it in terms of probability (the following numbers are arbitrary just for illustration). You need 32-33 starts for each spot in the rotation, but each starter has some probability less than 100% to actual provide this. Pivetta is the closest we have to someone with 100% probability due to a significant track record of both quantity and quality of innings, but even he is at say 90%. That would mean Pivetta is “worth“ 29 starts and you need to have the depth to cover the other 3-4 starts needed for that rotation spot. Do this math across all of your starter depth and it has to add up to 162.
Now someone like Sale might only be worth 50% or 16 starts, yet his status and contract all but guarantees him a spot if healthy. This is where having optionable depth like Bello is critical. But even Bello may only have a 50% probability of delivering 32 major league quality starts. You can quibble with the 50% number for Bello, but imo posters are assigning too high of a number here.
Which brings me to this. I agree that if the off-season pitching plan is to leave the starter depth as Sale/Pivetta/Whitlock/Paxton/Hill then Bello should be in Boston. But as I said in my first post, I expect them to add 2 established starters in addition to those listed.