3. Celtics’ small ball hasn’t worked
Game 4 will be the 10th straight game that Kristaps Porzingis has missed with the calf strain he suffered in Game 4 of the first round. That injury has pushed Al Horford into the starting lineup, and that lineup has been much better in the playoffs (plus 18.1 per 100 possessions in 195 minutes) than it was in the regular season (plus 2.7 in 311 minutes).
With Horford in the starting lineup, Luke Kornet was the backup center until he sprained his wrist in the first half of Game 2.
With Kornet out, the Celtics initially went to small lineups — with Tatum or Oshae Brissett at center — when Horford sat down. But those lineups haven’t been good:

So Xavier Tillman has been getting some minutes as the backup center, including more than six minutes alongside Horford in Game 3 (all of the two big minutes in the table above).
In the second half on Saturday, the only time there were zero bigs on the floor was the last seven seconds of the third quarter). The bigs were missed in those seven seconds because the quarter ended with McDermott getting a tip-in over Payton Pritchard to put the Pacers up nine.
Horford and Porzingis (when he returns) allow the Celtics to play big without sacrificing spacing on offense. Kornet (who’s listed as questionable for Game 4) and Tillman, not so much. However, the latter’s minutes were critical in Game 3, and we may not see much more small ball going forward.