Celtics fans must have felt a little uneasy about the 7-point lead their team held at halftime — given that BOS had hit 8 threes by then (on 14 attempts: 57%) and shot 52% overall. The focus of that uneasiness might well have been the Wolves' extremely easy penetration of, and scoring in the paint — MIN had 26 paint-points at halftime, 56 total for the game (on 28/49 shooting).
I.e., though the Celtics ended up with a good-looking Defensive Rating for this game, they did little to shore up their recent vulnerability in the paint. Minny scored more than half their points inside, and with relative ease.