Theis has actually played well in his four games. He's scoring at about the same rate as Timelord and Horford, he hasn't had a turnover yet, he's shooting 86% from 2PT and 50% from 3PT, and he's been the best rebounder that's currently on the roster (even better than Timelord). And, the team seems to have played well in those four games; he's got the best DRtg and best ORtg on the team.
So, if this is a "earn your minutes" situation, Theis has performed as well as can be expected. He has fouled too much, but that doesn't seem to outweigh his contributions elsewhere.