The Giants optioned Brennan to Triple-A Sacramento on Tuesday, Shayna Rubin of the San Francisco Chronicle reports.
Brennan's .325/.386/.500 slash line through 44 plate appearances in the Cactus League made a strong case for him to break camp with the big club, but he'll ultimately head back to the minors to begin the year. The 28-year-old outfielder has routinely batted above .300 at the Triple-A level and could earn a spot on the Giants' bench if he continues to swing a hot bat.