I am arguing with your concept and that "Well yes they’re not going to build up his arm for three years when’s he’s 40 years old to be a starter" statement. They don't care about building up his arm (to the point of having him work as a starter). Period. If he's good enough (miraculously) to hold his stuff over 4-5 innings, his control/command has improved significantly and the Cubs have a clear need for another starter at the end of the season then he's a SP. If injuries strike and they need another SP down the stretch (and most importantly) think Hagerty could do the job -- then they'll use him as a starter whether his "arm is built up enough" or not. The Cubs aren't going to "baby him" like what the Nats did with Stephen Strasburg back in 2012. I seriously doubt any of this will happen, but it has nothing to do with whether his arm/body has built up. Reliever is just the most logical role for him at this point due to multiple factors (max-effort delivery, control/command issues, mindset/makeup). EDIT: Also, it's important to note some pitchers have problems adapting to relief pitching. They have problems getting loose/warming up, get fatigued faster pitching every few days instead of every 4-5 days, and simply don't like the role. I doubt this is the case with Hagerty, but I honestly have no clue.