The interesting question for the Celtics is choosing their philosophy and eventually dealing with their wing logjam before issue solves itself.
If I assume they go small ball their lineup looks something like this:
Thomas - Smart
Bradley - Rozier - Smart
Hayward - Brown
Crowder - Tatum
Horford
Personally I feel like this way their defense will be not contention worthy, but we'll see...
I think the only way to contend with Isaiah Thomas is by having a good to great interior defender(switch capability or rim protector) - Either of Gobert, Davis, Gasol or Jordan and other candidates who'd emerge in that category include Embiid, Giannis, Porzingis, Towns, Noel, Capela, Adams, Turner
If they go with a more traditional lineup they'd need another big somehow and then their lineup would look like:
Thomas - Smart
Bradley - Brown - Rozier
Hayward - Tatum - Crowder
*** - Crowder
Horford
They'd need to use 1 or two wings and convert them into a big. Not a clean roster makeup right now even though with their depth and those juicy 2018/2019 first round picks they control they'd get a deal done