"I thought I was going to get a triple and it grabbed into me," Costa said. "So I just kind of cruised into second."
He came out of the game immediately and was replaced by Esteban German, who is basically an infielder who can play the outfield.
"I don't know how Costa is," manager Buddy Bell said. "I know there's a really good chance that Costa is probably going to go on the disabled list."
David DeJesus, the regular center fielder, is already on the DL with a strained left hamstring.
Not only was Costa hurt, but designated hitter Mike Sweeney left the game in the ninth inning with lower back tightness. He was
injured while dodging a pitch that the A's Kirk Saarloos threw for ball four.
Sweeney will be re-evaluated by the medical staff on Saturday.
Costa was scheduled for an MRI on his hamstring.
"It's sore right now and we'll see how it goes from there," he said.
Costa was doing a good job of replacing DeJesus, hitting .317 with three homers and eight RBIs. He had 19 hits in 19 games.
The most logical replacement would be fleet-footed Kerry Robinson, who is hitting .367 for Triple-A Omaha and has 12 RBIs and seven stolen bases in 20 games.
Other Omaha possibilities were not doing well. Chip Ambres was batting just .152 and Aaron Guiel just .200.
"It's just a string of bad luck," Costa said. "When it rains, it pours, I guess."
Dick Kaegel is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.