Zach Lowe points out that the Heat's unconventional lineups went a long way towards giving the Celtics fits.
The Point Forward: Small wrinkle sparks Heat
In their 99-90 Game 1 victory Sunday, the Heat dared Boston to either go small along with them by inserting Jeff Green at power forward or keep two big men on the floor and deal with the quickness disadvantage, weird matchups and unconventional defensive responsibilities that come with guarding a small lineup. The Heat went small, with only one true big man on the floor, for 15:18 — nearly a third of the game, and a much bigger chunk of time than Miami was usually comfortable giving to units of this sort against teams that prefer big lineups.
Zach points out that Miami outscored Boston by 15 points in those 15 plus minutes (he goes into detail on how they did it but it has a lot to do with LeBron James killing Jeff Green). it is up to Boston's coaching staff to figure out how to counter this strategy.