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.
Jason Vosler pinch-ran for Estrada and took over at shortstop. Leiter remained in the game for Chicago.
Estrada, 26, is batting .262 with nine home runs and 41 RBI this season.