"We gave him enough days off and he's feeling a lot better," Cubs manager Dale Sveum said.
Castillo said he's still a little tender, but doesn't want to go on the disabled list with so little time remaining in the season.
"I'm not 100 percent, but I have to get through the season," said the catcher, who feels it more when he's hitting than when he's making a throw from behind the plate.
Castillo, who has not played since Wednesday, went 3-for-26 in his last eight games, and was batting .265 entering Sunday's finale vs. the Padres.
Carrie Muskat is a reporter for MLB.com. She writes a blog, Muskat Ramblings, and you can follow her on Twitter @CarrieMuskat. This story was not subject to the approval of Major League Baseball or its clubs.