#3 Diego Castillo
Until the relief pitcher market works itself out over the next few weeks, it seems likely that Jose Alvarado will be the go-to man in the ninth inning, but other than that, it is uncertain who will be playing what role in the 2019 bullpen.
I believe that by the end of the upcoming season, Diego Castillo will have established himself as the perfect set-up man to Alvarado.
Castillo had a terrific rookie year in 2018, posting an impressive 3.16 ERA in 43 appearances. He had some midseason struggles that were quickly rectified by moving to being an opener. In 11 opens, he had a 2.65 ERA and straightened him out for the remainder of the season.
The right hander, who is only 24 years old, has electric stuff and owned a 10.32 K/9 last year. He is still a little rough around the edges, however, as he struggled with command throughout 2018 and will need to cut down on the walks in order to develop into an elite reliever, but he has proven to be a quick learner and is developing at a rapid pace, and so you should expect great things from him with an increased role in 2019.