Now, one intriguing deal, now that Indiana is thick at the 3 spot but thin at the 4, would be Baby/Scal for Posey and Rush, if Indiana would do it.
I think Indiana would do this deal and come out ahead talent-wise. Posey's biggest asset on that squad is his veteran leadership, and Rush is an inconsistent wing that will be average at best based on what he's shown in Indiana (his frustrating play on the court would harken C's fans back to the Tony Allen era). Though the Celtics get a bit deeper, I don't think they gain much from this trade as a whole.
Meanwhile, Davis gives them a Hansborough-esque clone at PF that can eat minutes and put up solid numbers when given the chance. The Pacers get more talent and consistency out of this deal and get what they need - a semi-legit big who plays hards and can start at PF most nights.
I don't think the Pacers will waive Posey, with Earl Watson leaving, Dunleavy on the block thanks to his EC, TJ Ford doing TJ Ford things, and Jeff Foster being a raging dick, they could use some veteran strength, which Posey has in spades. I think Bird regards him as the right type of player for his young squad.