The 22-year-old still finished with a team-high 24 points and 15 rebounds, but the shooting was too inconsistent.
Dwayne Wade also had 22 points in addition to Robin Lopez's 18 points.
"We're like this", Jimmy Butler said, balling up his fist to show the team's tightness after a regular season with occasional fractures. "That's where your confidence always comes from".
Now tasked with erasing a serious deficit, the Celtics will turn their attention to Game 3, which will tip off Friday night (7 p.m. ET, ESPN) at the United Center.
Isaiah Thomas led the Celtics with 20 points while Jae Crowder chipped in with 16.
They'll head home to Chicago with a 2-0 lead in the best-of-seven set thanks in large part to a vintage performance from Rajon Rondo. After losing in the first round of the playoffs the last two seasons many thought that this was the year they could win one maybe two playoff series. Boston's early rebounding edge evaporated just as fast, with all five Bulls starters grabbing an offensive rebound in the first six minutes of action.
It was so unacceptable that Stevens made it the focal point of his post-Game 1 coaching plan.
Chicago again used its big men to push the Celtics out of the paint and was quicker to the ball in a lot of 50-50 situations. That was one of a few lineup experiments from Brad Stevens, who has been resigned to throwing combinations at the wall and seeing what sticks. In fact, according to Avery Bradley, he could hear Rondo saying the Celtics gave up. "We knew their stuff, just like they knew ours".
"(Rebounding) was the game. Thomas missed 6 of 13 free-throw attempts, had two assists and committed five turnovers.
He scored 22 points as well as the clinching basket with 10 seconds remaining at home to the Bucks, while DeRozan had 23 points of his own. "I don't have any siblings, but I have some very, very close people in my life and I can only imagine how that would take your heart away", James said. "I felt like that in the first half".
WALTHAM, Mass. (AP) - In any sport, the one thing a team can't prepare for is the unknown.
Thomas is expected to fly to Washington state to be with his family and help complete funeral arrangements for his sister.
Bradley says it's important now to pick Thomas up on and off the court.
Now, obviously, the Celtics aren't making any excuses, as Al Horford told reporters "We're never going to make excuses, but this is hard".
Game 2 is tonight in Boston. Chicago's win also snapped the Celtics' six-game winning streak in such games.
Milwaukee will still fancy their chances heading back to Wisconsin for Games' 3 and 4 though, as their length again caused match-up problems for Dwane Casey's team.