Pachulia scored 10 of his 20 points in the fourth period, Milwaukee came up with 66 points from its bench and the Bucks scored the game's final 10 points.
The stunning outcome left Brad Stevens without a win in his first two games as Boston's new coach. Boston owned a 72-50 lead with 7 minutes left to play in the third quarter.
John Henson's layup with 2:27 remaining put Milwaukee in front for good at 97-96 and the Celtics could not produce another point. Pachulia scored six points during the game-closing 10-0 run.
It was Milwaukee's first win of the season.