There is a lot you can point to that went wrong or that could have been better in this game. I like to check the 5 man unit results and for this game, our red hot starters played 18 min together and had a NRtg of -6. Our ORtg was fine at 120 but this line up gave up 126. That is uncharacteristically high offense against our defense. The unit ended up just -2 in terms of actual +/- but still.
Dallas was very effective at continuing to do Pick and Roll until they got the match up they wanted. Often this was when Horford ended up on Doncic. This usually resulted in a double from us and they were good (at least in the second half) at making multiple passes out of the doubles and getting a decent look.
To me, this was a plan. Bring multiple pickers, force multiple switches, then bring Horford's man for one more pick, get Doncic isolated on Horford and either let Doncic take on Horford or if the double came, pass it around and force us to do multiple defensive rotations.