Santillan will start Sunday's game against the Rockies in Cincinnati.
As expected, the 24-year-old right-hander will temporarily slot into the rotation as a replacement for Sonny Gray (groin), who is expected to miss at least two starts while he's on the injured list. Santillan excelled over his six starts at Louisville this season, posting a 2.45 ERA, 1.09 WHIP and a 45:12 K:BB in 32.1 innings. He'll draw a favorable matchup Sunday for his big-league debut against a Rockies offense that ranks last in the majors with a team-wide 58 wRC+ away from Coors Field this season.