There are a few trains of thought when dealing with an unexpected obstacle in sports. In the Celtics case, it's the Bucks' length and size that gave Boston fits in Game 1. The Celtics set a franchise record of futility by only hitting on ten 2-point shots the whole game. Boston already starts two bigs in an NBA era where most teams only play one at a time, so trying to go bigger isn't an option. And in my opinion, even if it was, it would be a poor one.
A second option is to keep doing what you've been doing and hope that Game 1 was an outlier.