Chicago also reinstated catcher Welington Castillo from the family medical emergency leave list to serve as the 26th man in a split doubleheader at Detroit on Tuesday and then transferred left-hander Manny Bañuelos to the 60-day injured list.
The White Sox signed Santiago to a minor league deal on June 21. He had a 6.75 ERA and 1.88 WHIP in eight innings across eight appearances for the Mets earlier this season. He has posted a 4.50 ERA in 15 appearances, including 14 starts, at Triple-A. Santiago will start the second game of the doubleheader for the White Sox on Tuesday.
Castillo is hitting .176 with five homers and 16 RBIs in 46 games.
Bañuelos is suffering from left shoulder inflammation. He has a 6.40 ERA in 13 appearances, including eight starts.