Whether starting or relieving, since his debut in 2013 Carlos Martínez has been a valuable pitcher for the St. Louis Cardinals. Per Baseball Reference the value is quantified in the amount of 14.3 rWAR during that time. A bulk of that value has come as a starter — an expected result as he has amassed over 712 innings in that role as opposed to just over 152 as reliever.
And there-in lies the problem with Carlos Martínez, Reliever. He has certainly had success pitching out of the role, but he has had equal success as a starter. It makes the decision to put him in the bullpen for 2019 a puzzling one.