Cespedes was scratched from the lineup about two hours before the game, and within the next hour the A's had decided to place him on the disabled list and recall third baseman Josh Donaldson from Triple-A Sacramento.
"He took batting practice and was still feeling it a bit, so I wanted to be precautionary," manager Bob Melvin said. "We just can't fool around with it. He's a guy you don't want to string along like that, and we don't want him to fight through something like that. He's too important to us and it's too early in the season."
Initially the A's had announced that outfielder Michael Taylor was going to be recalled, but had they done that, Cespedes' DL stint could only be backdated one day, because Taylor had been in the Majors until two days ago.
By recalling Donaldson, the A's can make Cespedes' DL stint retroactive to May 7. He will be eligible to return on May 22.
Forst said he's not sure if that's realistic.
"I hope so," Forst said. "Again, this is a guy who swings pretty powerfully and needs to have that muscle working."
Cespedes said he first felt the injury about 10 days ago, but the pain went away and did not cause him to miss any time on the rest of the trip.
But on Tuesday, Cespedes aggravated the injury during batting practice and he had to be scratched from the lineup. He seemed to be improving, and just over three hours before the first pitch on Saturday, he smiled as he talked with reporters about how eager he was to get back in the lineup.
But Cespedes only made it through two of his normal four rounds of batting practice before stopping.
"The first round was OK, and the second round I felt it on one swing, so I stopped so it wouldn't get worse," Cespedes said. "It's not a bad injury. It won't be too long. I think I'll be ready soon."
Cespedes has five homers and team-leading 21 RBIs in his first season in the Majors. Rookie Collin Cowgill started in his place in center field.
Donaldson, who was expected to arrive in Oakland some time during Saturday's game, was hitting .447 in 13 games at Triple-A Sacramento after starting the season by hitting .156 with the A's.