Yabuta's bullpen spot was filled by right-hander Robinson Tejeda, claimed off waivers from the Texas Rangers on Tuesday.
Yabuta, originally signed to be a setup man, pitched mostly in middle relief. He had a 1-3 record and a 5.46 ERA in 26 appearances.
Early in the season, it was rough going for Yabuta. However, between May 9 and June 7, Yabuta was pitching better, allowing just one run in 11 2/3 innings in 11 games. But he'd slipped lately. In his last six games, he was 1-3 with a blown save, giving up five earned runs on eight hits and four walks in six innings.
"I think part of it was lack of consistency. He needs to continue to attack the strike zone in almost all counts, with the exception of the obvious 0-2," manager Trey Hillman said.
"In the meeting with him today, I emphasized the importance of utilizing the two-seam and four-seam fastballs, working ahead in the count and utilizing his split, his change and his slider as effectively as possible. I told him we expected him to get it going sooner than later."
A 12-year veteran of the Chiba Lotte Marines in the Japanese Pacific League, the 35-year-old Yabuta was signed last winter to a two-year $6-million contract plus an option of $4 million for 2010.
Hillman, who had seen Yabuta pitch during his five years of managing in Japan, gave the veteran the word about going to the Minors after Tejeda arrived for the game against the Colorado Rockies.
"I'm not going to say he was visibly upset, but he was disappointed, the same way that any player would be. He wants to stay at the Major League level," Hillman said.
Ron Mahay and Ramon Ramirez were doing most of the setup work and Yabuta's chances to pitch regularly faded.
"We want to provide him with the opportunity to throw every other day and sometimes, two days in a row, and I think that'll help his effectiveness and his control come back," Hillman said. "And when that happens, his confidence level will go back up, because he does have four Major League-quality pitches that he can use here."
Yabuta will join an Omaha team that returns home on Friday after a 16-game road trip because of the College World Series. It'll be a very different environment for him.
"It's difficult for a foreigner, but he's already had his American transition period and I'm hopeful that he doesn't have to now have an Omaha transition period," Hillman said.
Dick Kaegel is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.