It will be interesting to see how Udoka addresses the Theis issue. I can see three options:
1. Change the coverage scheme. Theis was playing drop coverage on the pick and roll and leaving Kyrie wide open. Maybe a different coverage is needed. Maybe Marcus needs to go over the pick and stay on Kyrie, or Theis needs to pick up Kyrie, which would open up a chance for Kyrie to drive. I don't think the issue here is 100% that Theis is physically overmatched - there might also be an element of Theis not understanding the coverages. John Karalis suggested that this coverage is similar to what Rob does, but Rob has the mobility to come up late and contest the shot from the same spot where Theis can do nothing.
2. Start Grant instead of Theis. Grant wasn't great in game 1 but he held his own defensively in some matchups, such as when switched onto KD. He has more mobility than Theis and knows the Celtics' system better.
3. Start White instead of Theis. Go for mobility over size and play Tatum at the 4. This was the closing lineup that won game 1. However, this might give Brooklyn a chance to try its "KD at the 5" lineup and it would creat substitution issues for the Celtics. With every rotation guard except Pritchard in the starting lineup, this would risk the Celtics getting into a situation where they need to give Pritchard minutes he should not get.
#3 is out. #1 is most likely, IMO.