Phillips was added to Boston's active roster on September 4. He made his Red Sox debut the following day, hitting a two-run home run in the ninth inning to carry the Red Sox to a 9–8 victory over the Atlanta Braves, completing a comeback from Atlanta's 7–1 and 8–7 leads late in the game. Phillips was the first player in Red Sox history to wear uniform number 0. Overall with the 2018 Red Sox, Phillips appeared in nine games, batting 3-for-23 (.130) with one home run and two RBIs. The Red Sox finished the year 108–54, clinching the AL East pennant. Phillips was not included on the team's postseason roster, as the Red Sox went on to win the World Series over the Los Angeles Dodgers.