Tatum will be the starting PF and play more than some there. Not sure why people don't get this. Tatum is the prototypical PF/big wing/stretch 4 in the NBA today. He will probably be playing at around 6'9" 220 this year, judging by the looks of him.
My guess is that the Grant/Juancho/Parker/Hauser(let's be serious, he will be in Maine all year) will be fighting for scrap minutes. Maybe only 15 MPG or so a game.
This always seemed obvious to me before that Tatum would start at PF, but I'm not 100% sold.
The Horford-Amir and Horford-Baynes pairings were starting lineup staples in some of Brad Stevens best seasons, despite the fact that many people wouldn't think it would make a lot of sense on paper before seeing it.
The league has more good bigs as starting PFs than people think these days, especially in the East. Sabonis, Giannis, Randle, Collins, Zion. Makes a lot of sense to start Horford at PF in those games, and maybe generally. Udoka said that Smart is the team's starting point guard. That makes me think Schroder is coming off the bench which doesn't leave me with a single obvious starting lineup where Tatum is the PF. I think Tatum at PF is a guarantee if Schroder starts, which I could see happening, but doesn't seem to be the case now.
I do expect Tatum to play approximately 25 out of 35 minutes a game at PF. Too much of our talent is 1-3 players to not play them and Tatum at PF. I see Horford with 14 min at PF (and another 14 at center), and Grant/Jabari/Juancho playing about 9 min combined. (The minutes per game of those bench players will get inflated way up at season's end due to DNPs and when starters miss games.)
Ultimately I won't be that surprised either way if Tatum is starting at SF or PF this season. But I'm leaning towards SF.