CBS is not prisoner to the standard positions of PF, C, SF, SG and PG. Those of you here still referring to them have not caught up to the new NBA and the changes. Bigs, Swings and Ballhandlers are what Stevens uses.
"I don’t have the five positions anymore," Celtics coach Brad Stevens said, per Kareem Copeland of the Associated Press. "It may be as simple as three positions now, where you’re either a ball-handler, a wing or a big.
In the book of Stevens, players generally fall into one of the four following categories:
Ball handlers – Typically played by the 1.
Wings – A hybrid between the 2 and the 3.
Swings – A hybrid between the 3 and the 4.
Bigs – A hybrid between the 4 and the 5.
This has been known for a long time since 2015, time to come out of the cave, perhaps
http://www.nba.com/celtics/news/sidebar/091015-Roster-Breakdown-The-Bigs
None of this is relevant. It doesn't matter if Stevens wants to make up his own positions, or doesn't even have positions at all. The only thing that matters, in terms of the debate on how to best use Tatum, is whether or not he can defend the 2nd biggest player on the court.
And while it does seem like the plan was to start Tatum as the PF, or biggest 'wing' per Stevens chosen vernacular, that lasted for a whole 5 minutes. And that was against Kevin Love, who isn't exactly a very physical player. After that, it's been Horford starting as the PF, or smaller 'big' per Stevens vernacular. Tatum has rarely been the 2nd biggest player on the court for the Celtics this year. He usually plays alongside two 'bigs' or alongside Morris, who typically takes the 2nd biggest player on defense.
But again, the question is whether or not Tatum can matchup against the opponents 2nd biggest player. If that player is bigger than he is, which they most likely will be, will Tatum be able to exploit his advantages more than the opponent can exploit their advantages?