I love this kind of discussion. TP to Tr1boy.
Reason is that, the starting unit has too many players with similar skillset.
Jaylen is more of a driver (‘fire’, in his word), while Jayson is more of a shooter (‘ice’) - but they’re both working on all three skillsets. It’s a good duplication to have!
You can’t have too many players who can create shots or make a play off the dribble!
Right now what CBS is doing is playing all the heavyweights together. Then extending guys like Walker and or Tatum over the limit, to make it up for the thin bench. Frankly at times, they look gassed.
Walker is playing 32.3 mpg. I wouldn’t call that “over the limit”.
Brown is getting more minutes than Walker.
Tatum leads the team in minutes at 34.2 - not onerous, but it’s worth asking why. One crucial thing, he’s getting developmental minutes to learn to deal with blitzes and doubles, which he’s seeing more of now. Very encouraging that his assist% has been creeping up. The work he did over the summer in shooting in the lane and finishing through contact needs a lot of game reps, but I for one think that he’ll get that clicking this season - and when he does, opponents can’t crowd him so much - and the offense opens up for everyone.
Both Jays are advancing as playmakers; the bigger picture is that they’re being developed as complementary threats to drive, make a play for a teammate, and shoot with range.
Celts leads usually suffer when the starting unit sits.
The starters are virtually never sitting at the same time. There are always at least two of the core five players on the court together.
What I propose is
Starting unit
C - Robert Williams
PF - Grant Williams
SF - Jayson Tatum
SG - Gordon Hayward
PG - Kemba Walker
Off bench unit
C - Theis/Kanter
PF- Semi
SF- Brown
SG- Smart
PG- Wanamaker/Edwards
Two non-shooters in the starting lineup? And at the moment, btw, Ojeleye is in the rotation, while Grant is not.
I understand where you are coming from. However inserting 2 "big" men in the starting unit that are not rotation level players should only happen if you are trying to tank.. 
Rob was in the rotation before he was injured.
So Theis is starting material all of a sudden but Timelord is not?
That’s what Brad Stevens thinks, evidently.
Theis provides very little on offense and while gives you some good moments on D, is not exceptional at that end either
It’s hard to agree with these two things, but I would say that his size is an issue against bigger opponents. Block % of 6.4% IS exceptional.