De Los Santos, who was called up from Double-A Pensacola, had a 1.54 ERA in 17 relief appearances for Pensacola after being claimed off waivers from the Nationals on July 19.
The Reds initially acquired Magill from the Dodgers in a trade for Chris Heisey on Dec. 2, 2014. He encountered arm issues, including undergoing Tommy John surgery in 2015. He had a 4.85 ERA in 58 games combined for Triple-A Louisville and Pensacola.
The players were summoned after Pensacola was eliminated from postseason play over the weekend.
Mark Sheldon has covered the Reds for MLB.