Bradley is nice but replaceable. I don't think the Celts are in "need" of a starter but an upgrade at that position would certainly not be unwelcome.
The story with Bradley is the same as it's been for a while: He's a pretty good shooter, though he tends to be pretty streaky. He's having one of his best years, and his True Shooting is still below 55%. He's a quality defender. He's averaging a healthy number of steals this year. He doesn't get to the line very often, he doesn't do much playmaking, he doesn't make an impact on the boards.
All in all, Bradley's a guy I like having on my side. There are a handful of starting shooting guards around the league you could replace him with and the team would be about the same. There are another 8-12, at least, that would be an upgrade.
When you consider that Hield or Dunn could be the BPA where the Celts pick in top 10 this year, the Celts will be in a good position to trade Bradley, or any of the other guards, in order to find an upgrade somewhere in the starting lineup.