Castro had singled with one out and one on in the first and was on second base when Jorge Soler singled to left against Brewers starter Yovani Gallardo. As Castro slid into home, his left ankle bent awkwardly underneath him. He was able to walk off the field under his own power, but was obviously limping.
"It just looked very awkward," Cubs manager Rick Renteria said. "Any time you see somebody cringe in pain, you grab yourself a little bit. Obviously, he walked himself off the field.
"He didn't want to come out of the game," Renteria said. "That's a good sign."
Castro did not return, and Javier Baez moved from second to shortstop.
"We need him in the lineup," Cubs catcher Welington Castillo said of Castro. "That's a guy who doesn't want to miss any games. I hope he's all right. We need him."
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.