Manager Bruce Bochy said the blister on Casilla's middle finger is just "a little better."
"With that said, though, we'll do all we can to stay away from him another day," Bochy said Saturday.
Casilla last made an appearance Thursday, blowing a save against Washington. In his last four outings, Casilla has blown three saves.
Romo, on the other hand, is 5-for-5 in save opportunities. He has pitched 25 innings this season and in that time has allowed just two runs and 11 hits. He's averaging 10.8 strikeouts per nine innings.
"Every time I'm in the game, I feel like that's my opportunity to save the game," Romo said after Friday's win. "I don't really look at it as a closer or whatever. I don't want that title. I don't need that title.
"We're a solid bullpen. Casilla's done a great job for us this year. ... Who doesn't have a rough patch during the season? We have all the confidence in the world in him. He's our closer. He is our closer. I'm just another guy in the 'pen that's trying to contribute."