Matsui left Tuesday's game with the Giants with a sore back, a malady that has plagued him from time to time throughout his career. He underwent an MRI on Wednesday, according to Cooper, who said after the game: "We're going to wait and see day-to-day, and see what [the MRI] says."
Matsui warned Cooper about his back soreness before the game started, but Matsui thought he could get through the game without any significant setbacks.
That didn't pan out. By the second inning, Matsui was out of the game.
Cooper was planning to give Matsui the day off Thursday, so he's eyeing Friday as the earliest possible return date for the second baseman.
"If he's not ready, we'll go to plan B," Cooper said.
Mark Loretta started at second base Wednesday. Geoff Blum is an option as well.
Alyson Footer is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.