This is the beauty of having multiple draft picks. You can be somewhat patient and see which player you like best, and can deal whoever else is left. The Celtics had this option in the 2007 trade that brought in Garnett, including Sebastian Telfair even though the Wolves wanted Rondo. Back in '99 the Lakers traded Eddie Jones (who was one of the best shooting guards in the league at the time) because they had as budding Kobe Bryant.
Let's say Rozier really develops over the next year, and it's clear he's one of the Celtics' long-term fixtures. You can then trade away Smart, or Bradley, or whoever, players you've already determined are valuable but not franchise-changers. Same could happen in reverse, of course (that is, Smart blossoms and you get something of value for as young Rozier).
You get the picture: not all of these guys will grow up with the team, which is fine because the others will be dealt away. The team just has to be patient and see who blossoms.