Cubs reliever Mark Leiter Jr., recalled earlier Saturday from Triple-A Iowa, hit Estrada with an 84 mph changeup, sending him to the ground immediately. Estrada was on his back and spoke to training staff, eventually getting helped to his feet and back to the clubhouse.
"He's still a little dizzy," Giants manager Gabe Kapler said after the game. "Some pressure on his left side. So he's having concussion-like symptoms. We haven't yet taken that step to diagnose him with a concussion, but good chance that's where we'll be and we'll figure out what to do from a roster perspective in the next few hours or tomorrow morning."
Jason Vosler pinch-ran for Estrada and took over at shortstop. Leiter remained in the game for Chicago.
"We didn't have any indication that he did lose consciousness, although his eyeballs weren't great," Kapler said. "He was pretty stunned."
Estrada, 26, is batting .262 with nine home runs and 41 RBIs this season.
The Associated Press contributed to this report.