Seattle manager Scott Servais said Santiago will start Sunday's game against the Angels, Ryan Divish of The Seattle Times reports.
With Justus Sheffield (forearm) and Justin Dunn (shoulder) recently joining James Paxton (elbow), Nick Margevicius (shoulder) and Ljay Newsome (elbow) on the injured list, the Mariners have been forced to shorten their rotation from six to five men. Santiago will pick up his first start of the season as a replacement for Sheffield, and the veteran southpaw should have a decent amount of leash, despite working out of the bullpen all season. He most recently tossed a season-high 76 pitches in a long-relief Tuesday against the Yankees, giving up three runs on seven hits and two walks over 3.1 innings.