Well, this thread went sideways in a hurry.
Minus Gordon things are going to look a lot different, with the main beneficiaries (in playing time) likely to be Tatum, Brown, Rozier, and Morris (once he's back).
In the short-term, I think Stevens will stick with Brown and Tatum joining Irving and Horford in the starting lineup, with the 5th starter being one of Smart, Baynes, and Morris depening on matchups.
It's possible that, down the road, Brown or Tatum might slide into a bench role, with Morris always starting, but I don't think that will happen for a good long while. Tatum is the team's leading rebounder right now (among guys who have played starter minutes); not starting him might mean that Baynes needs to start to avoid getting killed on the glass. Brown, on the other hand, is the only "big wing" on the Celtics who can long regular minutes at the 2, so if he moves to the bench, Smart - who has yet to show any chemistry with Irving - must start. For now, I think Stevens will have Tatum and Brown start until one of them plays their way to the bench - too much long term benefit to getting them experience to do anything else.
Baynes, I think, fouls too much to be a regular starter, so I think that, except for teams where it makes sense to match him up on a big C, he'll mostly come off the bench. Morris will probably come off the bench at first, which would leave Smart in the starting lineup:
C Horford
F Tatum
F Brown
G Smart
G Irving
Key bench guys, for now (at some point, Smart and Morris may flip):
G Rozier
F Morris
C Baynes
At this point, the 9th bench guy will probably be Shane Larkin, because he can run an offense. That means more small-guard-based lineups than we were used to, with Kyrie getting some minues at SG and Smart and Rozier getting some minutes at forward. That will hurt them defensively but may be necessary for the offense, at least in the early going.
The other guys (Theis, Ojeleye, Bird (2-way), Nader, Yabusele) will get their opportunities based on the needs that come up in particular games. Theis, and then Yabu, will be there when an extra big is needed - which will probably happen pretty frequently. Ojeleye and Bird are 3&D wings - one bigger, one smaller - who will get minutes when that is needed. All three of them are probably ahead of Nader and Yabusele at this point; Nader would play when a wing with more of an offensive game is needed - that's probably less of a consistent need at this point. Yabu just isn't ready for a regular role yet.
That's 14 guys, one more than can dress. The guy who sits will vary between Yabu (not ready/4th big on depth chart), Bird (2 way deal), and Nader (least needed with everyone healthy).
I don't rule out Bird playing his way onto a regular contract at some point, which might put Nader's roster spot in jeopardy if the team tries to add a vet. That's not assured though - with Smart available, Bird might not be needed.