If you read between the lines of what Doc Rivers said yesterday, you can tell they were already working on bringing in Barbosa.
"We're always looking. Every team is always looking. If nothing comes, does that bother us? No. It'd be great to get another guy at any position. Obviously, at [point guard] it would help, but we can't also forget that Avery [Bradley] is coming back too. We don't want to overdo the point guard thing."
Basically this is a lot like the draft in which you get talent first and then figure out how to fit it all together later. Is Barbosa a pure backup point guard? Nope. But having too many talented combo guards is a problem we're willing to live with.
I'm guessing that the team has seen enough of Terry's ball handling to feel comfortable with him bringing the ball up and they will get creative with letting guys like Barbosa and Bradley bring the ball across half court and initiating plays through Pierce, Green, and even the high post centers (KG, Darko, etc.).
Oh yeah, and since Rondo's going to be playing 35 min a game and more, we're not talking about a huge role here (knocking furiously on wood).