Maybe it's preseason. But it's been 7 full games, and he hasn't found anything that works. I don't need him to tell me that the Celtics are going to be a "slow starting team", though this excuse doesn't make it any more acceptable.
You don't really know that he hasn't "found anything that works".
He probably knows exactly which lineups have worked (and which ones haven't). That doesn't mean he should stop playing other lineups to see what _else_ works or not.
Yes, this is preseason! A new coach with a complete set of new players who have barely played with each other!
I don't believe he's managing these games to win at all. Just look at some of the 5-man units he's left on the floor in some of the close games. No defensive or offensive substitutions at all - even when the game situation screamed obviously for it. He did lots of that at Butler in close games so it's not like he doesn't know how to do that. He's pretty obviously just putting players into a wide variety of situations and seeing how they handle it (or not).
I don't expect Stevens to 'settle' on a defined rotation for some time - even into the regular season.