There are only so many out of bounds plays Brad Stevens will have the opportunity to call. The Celtics have to figure out a way to get stops because they have to get transition opportunities to score with any sort of efficiency. If anyone other than Isaiah Thomas is dribbling, they look like a college offense and even Thomas was, at times, reduced to throwing himself into the nearest defender and hoping for a referee to bail him out.
The Celtics were probably fortunate to give up only 102 points. You can maybe wave away the totality of sending the Hawks to the free throw line 35 times through references to late-game intentional fouling and referees with very inconsistent views on allowable contact, but the Celtics don't typically play defense without fouling. They have to be aggressive to force turnovers at their preferred rate. And I don't think you can sell any of the favored narratives used to pretend defenses have a significant influence on single-game three-point percentage. The Hawks 27 three-point attempts were generally taken by the players the Hawks want taking three-pointers, they weren't unduly contested, and they didn't overwhelmingly occur late in the shot clock.
I don't know what Stevens can do differently, given his personnel, to force the Hawks into lower-percentage field goal attempts. Even hoping the Hawks continue to miss shots is extra cold comfort when you've just let the Atlanta Hawks (the Atlanta Hawks!) grab 27% of their misses. That's an indictment of either your execution or your talent. Amir Johnson is, and always has been, a good player but Millsap and Horford are better than he is at everything he's good at. Stevens showed little interest in playing Kelly Olynyk significant minutes. Jonas Jerebko can only hope to out-Mike Scott Mike Scott*. Jared Sullinger? You can't expect a fat guy with a terrible haircut to accomplish anything substantial against Al Horford.
*Mike Scott played as well as you could hope, more than making up for Dennis Schroder's disappointingly passive play to give the Hawks a crucial seventh contributor. Scott also passed Schroder as the Hawk most likely to take a swing at Marcus Smart. Speaking of, I thought Smart, despite being +6 for the game, offers the greatest hope for improvement for the Celtics in Game 2 and beyond simply by not taking six three-pointers again and showing better assignment discipline when asked to guard Korver.
It's widely recognized going small helps the Celtics less against the Hawks than it does against most teams because the Hawks, since Tiago Splitter was lost for the season, already play 5 out all the time. Going small has less positive impact when Jae Crowder appears incapable of keeping Jeff Teague in front of him. Stevens can't even hide Isaiah Thomas on Kent Bazemore if Bazemore moves without the ball because Thomas will get caught ball-watching.
All the Celtics guards and wings (with the exception of the now, sadly absent, Avery Bradley) are drawn to the ball, leaving them susceptible to quick, constant, and sound ball and player movement. Which is understandable, given that I started this by talking about how they have to create transition opportunities to score consistently. Such is the stress of being a good team confronting a challenge directed squarely at what you believe makes you good.
To their credit, there's almost no chance that any doubt the Hawks create for the Celtics will adversely effect Boston's level of effort. Which forces the Hawks to apply relentless pressure to make the Celtics crack the only place they will, the only place that matters: the scoreboard.