"That will be worked out over the course of the year," Royals general manager Dayton Moore said.
Yabuta joins new Royals manager Trey Hillman, who returns to the United States after five seasons of managing the Nippon Ham Fighters of the Japan League.
The Royals' interest in Yabuta, however, came well before the club approached Hillman about becoming the manager.
"Louie Medina and Rene Francisco have been scouting Yasuhiko before Trey came aboard. They liked him and thought he could be a good addition to our bullpen," Moore said.
"When we signed Trey, he validated everyone's judgments."
Yabuta will be introduced to Kansas City at a news conference at 3 p.m. CT on Thursday. Hillman, who just returned from Japan, will attend.
A native of Osaka, Yabuta becomes the second Japanese-born player in Royals history. The first was pitcher Mac Suzuki, from Kobe, who was with the Royals in two different stints from 1999 to 2002.
Hillman's experience in Japan will help him communicate with the pitcher, Moore said. No interpreter is permitted in the dugout so Hillman or pitching coach Bob McClure will be on their own during mound conferences.
"We've talked about that," Moore said. "We can't bring an interpreter out there so Trey could make that trip to convey what Bob wants."
Yabuta spent his 12 years in Japan pitching for the Chiba Lotte Marines of Japan's Pacific League with Bobby Valentine as the current manager. His career record is 44-59 with nine saves and a 4.03 ERA in 343 appearances including 86 starts.
In the last four seasons, he's been almost exclusively a reliever with a 2.80 ERA in 222 games for Chiba Lotte. Last season, in 58 games, he had four saves, 38 holds, a 4-6 record and a 2.73 ERA.
Yabuta throws a fastball in the 88 to 92 mph range, a changeup, a slider and a split-finger that is effective against left-handed hitters.
"He keeps his fastball down in the zone and doesn't walk a lot of guys," Moore said.
Yabuta made a splash in the 2006 World Baseball Classic, won by Japan, when he struck out Alex Rodriguez, Derrek Lee and Johnny Damon in a 1 1/3-inning outing against Team USA.
Moore feels that the presence of Hillman helped Yabuta decide to cast his lot with the Royals.
"I don't know how much that factored in, but, face it, all of us have been exposed to something the first time, and comfort is something we're looking for," Moore said.
The Royals also are interested in Japanese starter Hiroki Kuroda, who is being courted by several teams including the Seattle Mariners and Arizona Diamondbacks.
Moore, involved in international scouting during his days with the Atlanta Braves, is anxious to take the Royals in that direction.
"It's been a passion and a goal to get involved in this market," Moore said. "The players from Asia are very talented and among the most fundamentally sound players around."
Mildren to Omaha: Left-handed pitcher Paul Mildren, designated for assignment on Nov. 20, has cleared waivers and was outrighted to Triple-A Omaha by the Royals.
First baseman Wes Bankston was claimed off waivers by the Oakland Athletics.
A move to clear space on the 40-man roster for Yabuta has not been announced.