Assistant GM David Forst said Ka'aihue felt something in his leg while running the bases on Friday night. Ka'aihue was initially in the lineup on Saturday as the designated hitter, but he was scratched less than two hours before the first pitch.
He was being evaluated by doctors before Saturday's game.
"We don't think it's a long-term thing," Forst said.
The injury to Ka'aihue was one of the reasons the A's recalled third baseman Josh Donaldson, instead of an outfielder, when they put Yoenis Cespedes on the disabled list.