There is no real clear choice for this. That is the problem. I don't think it should be Tatum though. I wonder if Semi Ojeleye may improve enough to be the best option. Don't get me wrong, I don't think he is a great option, we don't have a great option, (or even a good option) but he may end up being the best option.
I am assuming that Kanter will start at C so this is between Theis, Ojeleye, RWill, GWill, Poirier, and maybe Fall (he would be the C). Based on track record, Ojeleye may be the best of this meager lot. RWill, GWill, and Poirier have no NBA track record really. Ojeleye at least has an NBA track record as does Theis, just not much of a track record.
Overall is similar but Semi has probably shown a little more than Theis. Semi has had moments when he played good D and made some shots. If he can do that reasonably consistently, he will win the job and get 20'ish minutes a game (as will Theis likely).