Since Chris Sale is probably the easiest comp to make to Hader, Sale spent a season and a half in the Sox bullpen, then they converted him at the beginning of his 3rd season to the rotation, where he obviously did quite well for himself. Hader's numbers in the pen thus far are even more dominant than Sale's, but Sale also had more pitch types than Hader, whose FanGraphs "changeup" is actually still just his slider according to Hader. I'd be concerned about that, but (small sample size alert) Hader's numbers against righties have been just as dominant as against lefties, so it seems like that slider is working well against righties too (and the fastball is best described as unhittable at this point if you look at those contact % stats). Hader has also shown dramatic improvement in other areas that might've worried me about his role in the rotation even last year (sorry for lack of formatting): 2018 2017 (Majors) 2017 (AAA) 2016 (AAA) 2016 (AA) Pitches Per Inning 15.94 16.34 17.83 18.06 16.96 BB/9 2.50 4.15 5.37 4.70 3.00 In addition to being better than ever this year, he's putting up his best numbers of his career in K/9 and BB/9 (except for 5 games in low-A with the Orioles in 2012), and is also getting more efficient as he goes. A lot of the things that might've worried people about his inability to go deep in games might not be as big of a deal anymore. I know there's been some concern that he can't hold his velocity late into games but I'm not sure where that originates from (probably in the minors a few years ago), and that's just something you're not going to know unless you give him a shot in the rotation I guess. The only other concern I have is that we have plenty of potentially good to great starters lined up who deserve shots in the rotation, but I'm not sure that's a good enough reason to leave Hader in the bullpen when he has shown that he can be as dominant as this. While Chacin is still in the rotation, I'm not going to be worried about us having too many good starters to not give Hader a chance.