This game was very different from game 1 in one regard, how the big rotation worked out. In game 1, we did much better with 1-big line ups (+27/31 min) than 2-big line ups (-10/16 min). This game, we were +11 with 2-big line ups in 30 min and -2 with 1-big line ups in 18 min. These +/- results change every game, they can be all over the place, but in this case the big change was Tatum. He brings enough size that you can "get away" with playing with 1-big. Without Tatum, you need 2-bigs.
The result of being able to play Tatum and only 1 big is that you can get more skill on the floor. Instead of Horford (for example), maybe you have Holiday or White on the floor. Tatum gives you both, the defense and rebounding of a PF and the scoring creation of a wing. His versatility is so valuable.
In this game, Brown was really efficient and made up for the scoring we lost without Tatum, but he is not able to replace his size. Mazzulla had to play more minutes with 2 bigs (30 min vs. 16 min) because he didn't have Tatum. That is the difference that Tatum makes.