Santillan was selected by the Reds with the 49th overall selection in the 2015 draft out of high school. After a slow start to his professional career, Santillan started the 2016 season with short-season Billings. In 2015, in the Arizona League, Santillan posted a 5.03 ERA in eight appearances (seven starts). He had strong strikeout numbers put also struggled with command. The 6-foot-3, 240-pound Santillan has a fastball that sits in the mid-90s, but can touch the upper 90s. He also has a strong curveball in the mid-80s.