The Pacers were more difficult to defend than Dallas IMO. Luka is the best offensive player obviously, but beyond that Dallas doesn't have the threats that the Pacers did.
TJ MConnell was a constant threat with his penetration and midrange game. Nembhard played with extreme confidence, and quite frankly was much better than Kyrie Irving has been thus far. Turner disappeared at times, but put together some big scoring spurts. So on and so forth. Plus the Pacers played much faster and freer. Better ball movement and man movement.
I don't know how much adjustments Dallas can make to pick the pace and play better. They're struggling to score because they're grinding against the Celtics half-court defense all the time. And they don't have the personnel to stop dribble penetration.