I consider Tatum a wing (at best a small-swing) and do not want to see him playing out of position (as a big) for the majority of his time. I am fine with some change of pace small ball line-ups or specialized match up line ups at the end of games.
Your concern is understandable, and hopefully he's put on some weight.
If he's playing mostly wing, he's got a lot more competition for minutes. In addition to Hayward and Brown, Marcus Smart gets a lot of "wing" minutes (whether or not you call him a wing, he was almost always on the floor with one or both of Kyrie or Rozier last season). So substantial swing minutes for Jayson Tatum are a done deal.
I'd point out that it works both ways - the opponent has to guard him, too, and there are few players who can both keep up with him and have a size advantage.
I am not sure concern is the right word but I just see it as bad basketball strategy to play Tatum out of position. Would GSW want to play Durant at PF? Durant is bigger than Tatum will ever be, even if he fills out (which I am sure he will some). Why would we want Tatum to be Rudy Gay trying to be a PF (who is also bigger than Tatum)?
Again, I am not saying that the Celtics will see it the same as me, I know my thinking is outside the mainstream, but keeping Tatum at wing will mean playing Morris, Theis, and Baynes more, while Smart and probably mostly Rozier less. There are plenty of wing minutes for Brown, Hayward, and Tatum at wing, especially with Hayward coming off such a serious injury.
Plus, Rozier and Smart will get minutes when Irving takes games off or quarters off resting his reconstructed knee. It will all work out. Along the way, Rozier may get traded or someone will get injured or...