The Cardinals acquired Masterson from the Indians leading up to the non-waiver trade deadline in exchange for minor-league outfielder James Ramsey. In six starts for St. Louis, Masterson has logged an ERA of 7.90 and a K/BB ratio of 2.00. He's averaged less than five innings per start over that span. Masteron's long-standing platoon issues -- in part a function of his low-slot delivery -- may indeed make him a better fit for relief work, at least right now.
As Hummel writes, Marco Gonzales or Tyler Lyons will likely replace Masterson in the rotation starting Thursday against the Brewers.