Theis was one of four Boston reserves to reach double figures. Marcus Morris had 15 points, Marcus Smart tossed in 12 and Terry Rozier chipped in 11 as the Celtics bench outscored its counterparts 65-21. Smart had missed the previous 11 games with right hand lacerations.
Boston made 17 of 39 3-point tries, while Detroit knocked down just 10 of 33.
The Celtics outscored the Pistons 38-21 in the second quarter to gain a 61-49 halftime lead. Boston did a majority of its damage in the first half from outside. It made 11 of 21 3-point attempts, while Detroit connected on 6 of 15 attempts from beyond the arc.
Theis, an undrafted rookie from Germany, equaled his career high by halftime with 12 points. He made both of his threes and a putback layup during a 13-5 Boston run to close out the half.
The Pistons didn't have anyone in double figures before the break.
Detroit tightened its defense and chipped away in the third, pulling within four points. Boston scored seven unanswered points late in the quarter, including five by Irving, to make it 86-75. James Ennis' dunk lowered Detroit's deficit to single digits entering the fourth quarter.
The Pistons then went scoreless for more than 4 1/2 minutes at the start of the fourth as the Celtics extended their lead to 16. Morris hit a 3-pointer and Smart converted two layups for a 93-77 lead. Morris drained another three midway through the quarter to nudge the lead to 20, 99-79.
Boston clinched the season series 2-1.
--Field Level Media