To make room for Santiago on the 40-man roster, the White Sox transferred left-hander Manny Bañuelos to the 60-day injured list.
Santiago, 31, was signed by the White Sox as a Minor League free agent on June 21. He went 4-5 with a 4.50 ERA and 71 strikeouts over 15 games (14 starts) this season between Triple-A Syracuse in the Mets' organization and Charlotte. Santiago also made eight relief appearances with the Mets, going 1-0 with a 6.75 ERA before being designated for assignment on June 15.
Santiago spent the 2018 season with the White Sox, going 6-3 with a 4.41 ERA and 103 strikeouts over 49 games (seven starts). He originally was selected by Chicago in the 30th round of the '06 Draft. He is scheduled to start the second game of the doubleheader.
Castillo, 32, was placed on the family medical emergency leave list on Friday. He is hitting .176 with five home runs and 16 RBIs in 46 games this season.