You really need to stop thinking of this team having 2 units. Stevens isn't like that. Most likely Stevens will have 2 starters, them being Smart, Kemba and the Jay's, playing with 3 bench players. There won't be a "second unit". Just guys in the rotation that best help the two starters that will be on the floor.
Theis and Thompson starting apparently
That's great! I want to see whether those 2 can play together well. If they can it becomes much easier to fill in the rest of the rotation. Apparently Smart/Brown/Tatum/Theis/Thompson starting? That's a very strong defensive line-up. We can't really outshoot the Bucks anymore, so maybe we can make it a grinding?
I'd like to switch Teague and Smart next time though. Add some playmaking and shooting with the starters and bring some defensive stability around our young bench. Below would make the most sense to me until Walker returns.
Even if Stevens doesn't really play two specific separate units he still needs to have guys play those bench minutes. We can't play Tatum, Brown, Smart 40+ minutes a game (at least we shouldn't). When Thompson and Theis start that makes it just easier to play Grant and Robert Williams a lot of minutes.
And to me these combinations matter (which follow from the starting line-up). If you just start Theis as your only big for example (with Tatum PF on paper), example: Teague/Smart/Brown/Tatum/Theis. You can't just substitute Thompson (first guy of the bench) for Tatum and then follow that up with Rob Williams for Theis, because Thompson/R.Williams is probably not a great combination together.
The Celtics don't have a very versatile group, so I think it's really important to plan your rotations and make sure you get the right combinations of players on the floor. And that starts with your opening line-up.