This is not that complicated, and it is also not about effort. It is about the stupid lineups the Celtics have on the court. Every coach in the history of basketball was not wrong for playing a Center on occasion. The Celtics got Kanter for the playoffs, so use him in the playoffs.
Stevens almost lost to a star-less Raptors team that had a mediocre offense, and now he has gotten himself down 0-2 to the Heat by playing a 6'8 rebounder and hustler at center against an all-star. I like Theis' heart, but he is not a big time player and should not be starting and playing 30 minutes in a Conference Final. Adebayo can play hero ball in the zone because he doesn't have to worry about Theis' offense at all. That takes away penetration, and enables the Heat to play more aggressive perimeter defense because they have Adebayo behind them to erase any mistakes.
If Kanter was on the court more, Adebayo would have to defend him, which would open up penetration. Kanter would also enable better ball movement because he could flash to the high post as a passing target to get the ball in the middle of the zone. Then the top of the zone defenders would have to drop back to help, leaving Smart, Brown and Tatum open from 3. A decent inside threat creates more and better 3 point opportunities.
I have lost a lot of respect for Stevens in these playoffs. This is stuff a middle school coach could figure out.