I think Rozier's ceiling is probably as a strong, backup, PG who can do a little bit of everything good but nothing great. Unfortunately, with the best players in the upcoming draft being guards or wing players there will be a serious roster crunch next year.
If the Celtics take Fultz or Ball, or even Jackson, they will get immediate minutes like Smart and Brown did before them. If the pick is Fultz or Ball, they immediately will step into Rozier's slot. If the pick is Jackson, he will be the backup SF, moving Brown to the backcourt and dropping Rozier from the rotation as well.
I think both Rozier and Bradley will be roster casualities possibly as soon as this summer or possibly as late next year's trade deadline. Rozier has a future in this league, not one as a star but as a player, but I just don't see it being with the Celtics.