I see three tiers of strategy on defending Doncic. Who starts out on him is only the first tier. My guess is that Holiday will be on him the most, but Brown and White will probably start some possessions on him as well.
The second tier of the strategy is the man that is brought into the play by the screener. What DAL will want to do is have Porzingis or Horford on Gafford or Lively, and than that will be the screen combination. If we switch, Doncic ends up with Horford or Porzingis. What the Celtics have been doing so far in the playoffs is putting Tatum on the guy that is most likely to be the screener. You may see Tatum on Gafford and Horford on Washington, for example. Then if Gafford screens for Doncic, Tatum can just switch.
The other nuance to getting the screen combo the way you want it is that DAL can do an off ball screen to try and get Tatum off of Gafford, to get that match up the way they want it, before coming to screen for Doncic.
The third tier of the strategy is whether to switch, blitz, double, drop, whatever, at the point of attack. This will depend on who the defender is that gets pulled into it by the screener, as much as it is about how is on Doncic to start. I think the best way to handle this is switch unless it is Porzingis and maybe Horford, and if it is a big, just drop and take your chances that Doncic takes a 3. That is better than letting Doncic into the lane where he has the triple option of shoot, pass, or get fouled or be iso'ed on a big. But you need to prepare so that you can change it up. You probably aren't going to do the same thing all game or all series.