I think if he finishes strong, he'll consider coming back next year.
They already updated his wiki:
Phillips was added to Boston's active roster on September 4[39] and made his debut with the Red Sox the following day. In that September 5, 2018 game, he hit a 2-run home run in the 9th inning to carry the Red Sox to a 9-8 victory over the Atlanta Braves, leading the team to its 97th win of the season, completing a comeback from Atlanta's 7-1 and 8-7 leads late in the game.[40]