On opening night last season, the starters were:
Irving/Brown/Hayward/Tatum/Horford...
So already Tatum had won the starting spot, in his rookie season. A year later, he has more than justified Brad Stevens' faith in him. But after Hayward went down, by far more minutes went to lineups with two bigs, and especially the bulk of the starters' minutes. Do you go with what worked last season? It's hard to argue with ECF game 7.
Hayward was out for a whole year, which is an eternity in the NBA. Many fans have argued that he should be eased in gradually for that reason; and with his shooting and scoring he fits the classic sixth man profile, providing scoring punch off the bench. Of the scoring leaders in the league in the last few years, he is one of the most likely to facilitate shots for teammates; so he's a good choice to come off the bench and elevate the offense of bench players.
The "five out" potential of that opening night 2017 lineup, where everyone is a "triple threat" (to drive, shoot with range, or make a play), has to be enticing for the coaching staff. And while there's no denying that Baynes has carved out a place for himself (not to mention a new contract), isn't elevating the offense a priority? The two-big lineups were outstanding defensively, but Boston was middle-of-the-pack on offense last year.
Marcus Smart has come off the bench most of his young career, but at the same time he's consistently gotten starter's minutes. Last year was typical; he was fifth per game on the team. Most commonly he came in for Baynes, with Tatum becoming the swing in a smaller lineup; or he and Morris would come in for Tatum and Baynes, again in a smaller lineup with Morris at swing; or, less often, he'd come in for Tatum in a two-big lineup. Bottom line: Smart is the incumbent sixth man.
Of the likely sixth men, that leaves Marcus Morris. He might be the rotation player with the least certain future, given that he has one year left on his deal and has overlap with Tatum. With Hayward returning, Tatum will probably get more minutes at swing and fewer at wing; Morris openly wondering earlier this summer where he "fit", going forward, shows that this potential minutes crunch has not escaped his notice. But despite dealing with his injury situation, requiring him to be shut down for a second time in January/February to rehab completely, Brad kept getting him on the floor; per game, he was sixth in minutes - by that measure, at least, he was the sixth man last year.
Anyone else? Rozier?