I love Barea, but I think the Celtics are too committed to their switching defense to swap Rozier for a guy as small as JJ. Even when Rozier isn't hitting shots, he's still skying for rebounds and playing solid defense with an ability to guard the 2, which allows Brad to use the three guard lineup - going small without sacrificing defense. You can have Rozier and Larkin on the court at the same time. You can't do that with Larkin and Barea. Teams can already look to post up Larkin on a switch, they'd eat us alive being able to go one-on-one against both him and JJ... we'd have to switch less, which would mess with the success of our defense and the versatility of our lineups.
Love JJ, but I don't think adding him to this roster would fit with Brad's schemes.