If the plan is to [possibly] flip Brown and assets in a bigger trade after the season (AD, anyone?), then he really needs to be starting. I like what Smart brings to the team, but Brad is right - he is best used as an energy guy and distributor off the bench. He orchestrates the 2nd unit and is the identity behind that unit.
Forget the playoffs for a moment, the starting 5 of Irving, Brown, Tatum, Horford, and Baynes was fantastic last year. While I don't blame him, Hayward threw a wrench in how well the team ran (since they did it all without him). If he were Kawhi, it would be a different story - he clearly needs a lot more time to adjust and get some semblance of athletic ability back.
I would just run with that line-up for a while and let the bench do their thing. Smart, Rozier, Hayward, Morris, and Theis is a ridiculous bench line-up. If Rozier can't adapt, then deal him, but demoting Brown - our most versatile and athletic defender who showed a lot of growth last year - doesn't make the most sense to me.