Fulmer was originally scheduled to pitch Wednesday at Pittsburgh, but instead will go Friday against Texas. Manager Rick Renteria said Tuesday the team wanted to get Fulmer some extra time in between his starts.
The 24-year-old Fulmer is 2/3 with a 6.23 ERA in eight games, seven starts, this year. He lasted just 1 2/3 innings in his previous outing against the crosstown Cubs on Friday, yielding five runs in an 11-2 loss.
Fulmer was selected by Chicago with the eighth overall pick in the 2015 draft.
Instead, Santiago will start Wednesday. He has made two starts this season, going 0-1 with an 11.25 ERA. In nine appearances in relief, he has a 3.26 ERA. His longest outing of the season is 4 2/3 innings.