Santillan was designated for assignment by the Reds on Tuesday.
Santillan missed the first three months of the 2023 campaign due to back and hamstring injuries and has posted an ugly 9.26 ERA across 23.1 innings of relief at Triple-A. The 26-year-old right-hander showed a lot of promise as a rookie in 2021, but he is now off the 40-man roster in Cincinnati.