De Los Santos, 23, was 1-1 with a 3.67 ERA and five saves over a combined 29 appearances for Triple-A Syracuse and Double-A-Harrisburg. In his 34 1/3 innings this season, he's allowed 34 hits and 23 walks with 39 strikeouts and a 1.66 WHIP.
Originally in the Rangers organization, De Los Santos was acquired in a trade from the Nationals for lefty Ross Detwiler in December 2014. He made two big league appearances for Washington in 2015 and had a 5.40 ERA over 1 2/3 innings of work.
Mark Sheldon has covered the Reds for MLB.com since 2006, and previously covered the Twins from 2001-05. Follow him on Twitter @m_sheldon and Facebook and listen to his podcast. This story was not subject to the approval of Major League Baseball or its clubs.