Contreras was optioned to Triple-A Indianapolis on Thursday, Jason Mackey of the Pittsburgh Post-Gazette reports.
Contreras served as a long reliever for the Pirates early in the season, and he logged a 3.52 ERA and 0.91 WHIP while striking out in in 7.2 innings in three appearances. However, he'll head to the minors after right-hander Chase De Jong's contract was selected Thursday. Contreras has served mainly as a starter in the minors and should spend time in the rotation with Indianapolis.