Horwitz will start at first base and bat sixth in Sunday's game against the Mets.
Heading into spring training, the 28-year-old's role was somewhat muddled following the offseason additions of Marcell Ozuna, Ryan O'Hearn and Brandon Lowe, but Horwitz looks to have settled into a strong-side platoon role at first base. He's occupied the position for the Pirates' first two games against right-handed pitchers, with O'Hearn spelling him Saturday when the Mets sent lefty David Peterson to the hill.