With a runner on first base, Eckstein, the second batter in the Padres lineup, squared for a bunt but pulled the bat back as the pitch approached. The Zambrano pitch, a 91-mph fastball, hit Eckstein in the upper chest and sent him to the ground.
Eckstein was attended to by Padres trainer Todd Hutcheson and manager Bud Black. He stayed on the ground for a few minutes before being helped to his feet. He stayed in the game but left after the bottom of the first inning.
Eckstein was replaced at second base by Edgar Gonzalez.
"He's doing fine. Last night I saw him with ice packs on his shins and shoulders. Tonight, it was on his chest," Black said. "I think he's going to be fine. That shakes you up a little. He was a little light-headed on the bases."
Eckstein was coming off a rough game on Thursday where he was hit in the left elbow by a pitch in the ninth inning with the bases loaded. That forced in the tying run in a game the Padres won, 3-2, when they scored two runs in the inning.
He also, on separate occasions, fouled a ball off his right knee and left knee and was also knocked to the ground by San Francisco's Pablo Sandoval, who was attempting to break up a potential double play.
Corey Brock is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.