I have a high amount of confidence Horton pitches in the big league pen this year, potentially meaningful innings too. We know he's probably not ramped to start the whole year, we know Jed really likes breaking in pitchers via the bullpen, and while there are promising options for MIRP(Smyly/Assad/Wesneski) we shouldn't have high confidence that for whatever reason(trade, underperformance, injury) there won't be a need in that role. Caveat: Counsell may lean less heavily on MIRP than Ross so maybe that part matters a bit less. I think the telltale sign for Horton is how many net SP they add relative to today's roster, inclusive of Assad/Wesneski/Brown. If it's zero then they probably are prepared to let it ride with Horton starting games through the season, if it's two he's probably ticketed for the pen, if it's one then it probably depends on the details.
Brown I just don't really know. Don't know if he'll be in the organization in March, don't know how he'l bounce back from his injury and if his AAA walk rate will calm down, don't know how many innings the org wants to get out of him this year. I suspect that he's in a bit of a bizarro goldilocks zone where he'll quickly be surpassed by Horton if the team wants impact, but won't show enough improvement in command to be near the top of the pecking order for reliability relative to the trio above or even other AAA RP options(Little, Roberts, Palencia, etc). So his fate may be dependent on an uncommonly good outcome from him individually or a series of negative circumstances from others.