Prior to his MLB career, Yabu pitched for the Hanshin Tigers of Nippon Professional Baseball from 1994 to 2004.

He pitched briefly in the Mexican League with the Potros de Tijuana, then sat out the 2007 season.

Yabu signed a minor league contract with an invitation to spring training with the San Francisco Giants in December 2007, and made the 2008 Opening Day roster.

During his year with the Giants, he had an ERA of 3.57, allowed 33 runs, and struck out 48, in 68 innings pitched throughout 60 games, with a win–loss record of 3-6 for the season.

On May 30, 2008, Yabu entered a game against the San Diego Padres as a reliever for the Giants with the score tied and with runners at first and second base, with nobody out.