The team as currently constructed has 5 players whose natural positions is PG, SG or SF, and that arguably have starter level talent; Walker, Smart, Brown, Hayward, Tatum. Then we have about 5 bigs who will see regular rotation minutes; Kanter, Theis, Ojeleye, RWilliams, Poirier (I am leaving out GWill as a rookie) where only one of them has any real track record as a starter.
I think these are the 10 players who will see the vast majority of non-garbage time minutes. Last season, the Celtics played about 2/3 of the time with two bigs on the court (I am including Morris as a big), with the most used line up by far being Irving-Smart-Tatum-Morris-Horford (495 min). So was that the "first" unit? That was not usually the starting unit or at least not always.
So after explaining the context of my mental construct of what "first" unit means, I think the "first" unit (not necessarily the starting unit) may well mirror last season and be Walker-Smart-Tatum-Theis, Kanter so the "second" unit would be the remaining 5 of Brown-Hayward-Ojeleye-RWill-Poirier, which isn't really a unit at all.
Now if you look at this another way, you could say who are the 5 players that play the most minutes and then 6-10 in terms of minutes. It turns out that the top 5 unit from last season also played the most minutes as individuals. This upcoming season, the 2nd big role may be more split up and spread out as opposed to Morris getting most of those minutes and Brown or Hayward may get more minutes than Smart (or similar minutes to Smart) but I think in the end, things will be pretty similar to last season for the main units usage.