All great choices. People will say Tatum can't play guard, he can while he's skinny. My vote is for the Smart Irving backcourt, we will see this lineup to closeout games. Although I am a big fan if Starting Brown because he won't have to do too much playing with the big 3. He can focus on defense and rebounding and hitting wide open shots, because Jaylen will be left open all day long playing with the big 3.
Brown is most likely. Brad said recently that he needs to be "a lock-down defender" - clearly Hayward is not going to be the designated wing stopper. But Jaylen'll be more than a spot-up shooter, too. He'll get more responsibility to create offense early on, and his role will expand.
I'm not sure what it would mean for Tatum to play guard. Meaning: he's going to guard DeRozan and Klay and Avery Bradley? I'm skeptical. No way he starts in the beginning of the year at
any position; but probably similar to what we saw this past season with Jaylen, he'll be worked into the rotation pretty early.
Having Brown in the starting lineup with Morris allows the Celtics to play 5-out and open the lane for Irving, Hayward, and Brown to drive. That's a juicy prospect, and I think that it means that Baynes and Smart are not starters.
Brad loves Smart. He was used as a sixth man last season (though he got starter minutes), and I'd bet that his role will be similar, allowing Irving to play off the ball.
Then again, everything has changed, hasn't it? You can count on the generals, they say, to be fighting the last war, at least until they get their asses whipped once or twice. I'd bet that Boston's high-octane offense will lull the roster into a false sense of superiority, and they'll have to look in the mirror and get real about defense sometime in January.