Theis is a 5, and I wouldn't expect otherwise. He's short but has bounce, plays hard/aggressive and didn't really get pushed around that badly. He played 2 mins with Baynes and 23 mins with Horford all of last season, so I wouldn't expect to see him play the 4.
If teams go double BIG (like Indy, Phila, Tor, Orlando or Mil) there is an outside chance we see him on the floor with TL? I really have no idea how Brad is going to thwart/address that?
Theis shot 75% on FTs and 39% on 3s in limited minutes. IMO He always played a hair tentative since a quick hook awaited any errors. A little rope may serve him well.
For what it's worth, his advanced numbers were very good in limited play (14mpg) last season. He had a positive OBPM, DBPM, VORP, ON/OFF, ORtg/DRtg.
Play him 20mpg and let's see what we have, if he sucks throw him in the filler pile after Dec 15
https://www.basketball-reference.com/players/t/theisda01.html