You're essentially forced to trade 1 of Irving, Rozier, or Smart. The timing of Smart's contract coming up first might mean they re-sign him, especially if they decide to keep Irving, because he's the better defensive complement. Rozier has a year left on rookie deal and then restricted matching control that will be super coveted.
I would keep Irving because elite talent, especially 1v1 creating, will be needed in the Finals. He's also only 2 years older than Rozier, only 26. Rozier might get to 80-90% of Irving, but that 10-20% marginal difference is the most expensive and most coveted thing in basketball. Plus with Ainge's track record and Stevens PG friendly system and development, I'd trust them to groom a replacement on the rookie scale or cheap FA. You'll have Smart to handle some minutes at PG anyways during development.
So I'd trade Rozier for a pick and/or someone with another year on a cheap contract (like they did with Bradley to Marcus Morris), possibly a PG entering year 2 or 3 like Tyler Ulis + pick (have heard Phoenix
rumbled as a Rozier destination). If they don't get a PG in return, they could try to sign someone in FA (dare I say Shabazz Napier would fit very well in the Shane Larkin role and would probably love to come to Boston????!!?? Also Jeremy Lin for same reasons) assuming their cap status allows it, if not then someone with their late 1st.