Barbosa is an upgrade of the spare guard over the two NBDL guards they just waived in terms of talent and experience. If pressed into service, he will help the Celtics, but there are better options on the team already.
This. He is a GREAT 5th guard, but much more iffy when he gets further into the rotation.
Really though, I think the key to the OPs question is that I don't think Bradley has all that much value. His resume is just very iffy, when another GM looks at giving up value for him.
(Disclaimer, this is going to sound overly critical, but GMs ARE overly critical when it comes to investing significant assets on trading for a player.)
He looked like a terrible player for 3/4 of his NBA career to this point, and then put together a nice run of a couple months last year, where he showed he is an elite defender, and can be passable on offense when completely ignored. For much of that stretch he shot at a level that he has never shot before, and it is very questionable whether he will again.
He also is an undersized SG, who had two separate shoulder injuries that needed surgery last year, and has had other injuries in the past that might suggest he could struggle to hold up facing guys much bigger than him night after night.
Now, he also has a high upside. They know he is an elite level defender...although it remains to be seen what position he should defend going forward. He is at his best defending PGs, but since he is not a PG offensively, it makes for a tougher job to find a backcourt mate, because ideally you would want a guy who can run the point, but defend the SG. Now, his athleticism does allow him to defend most SGs, but that does take away some of his effectiveness pressuring the ball, and it also brings into the question of his durability, getting beat up by bigger guys night in and night out.
Offensively, there is also a lot of upside. His quickness is a real asset, and he showed that he has at least a solid shooting stroke (potentially better than average, but it remains to be seen what a larger sample size, and more pressure on his shooting will hold), and he is a good finisher at the basket, with some slashing ability developing. Mostly though, his offense is still a big questionmark, because of the way he was defended last year, and the small sample size.
So, when a GM tries to give him a value, I just don't think they can count on him as anything more than a defensive role player, with potential to be more, but also some red flags.
And for the C's, I think he represents greater value. Since they don't have to give up anything to get him, then it is all upside for them. And it is going to take a lot more value to pry him away, than I think anyone will be willing to give up.