Because of the future aspects (i.e. Rondo's youth, manageable contract, etc) I wouldn't do the deal as is. That said, I pretty strongly disagree with those that say this wouldn't be an immediate help to the team.
I understand chemistry issues with big in-season moves, but honestly I don't think it would apply at all here. This Celtics team has already been conditioned to play off-the-ball while Rondo creates and distributes, so I don't see where the learning curve would be that steep going to Nash.
But on the other hand, Nash is WAY better as a finisher than Rondo. I mean, orders of magnitude better. And that improvement would be essentially directly applicable, as Nash can and would murder the defenses that are designed to play off of Rondo's shooting touch.
And defensively I think the impact is minimal. Rondo's disruption on the perimeter is cool, but in the scheme of things doesn't have a huge impact on our defense as a whole. Our starting unit with Nash would still be extremely strong defensively. And with Perk looking at a come back within the next month, Jermaine's loss is stomachable as well.
In a lot of ways, Nash's offensive impact is like a mirror image of KG's defensive. With a straight Rondo for Nash swap I think the Celtics would shoot to the top of the offensive e4fficiency list, and have a strong shot at finishing #1 on both offense and defense. Would definitely make the team better this year, IMO.
ETA: Oh, and I didn't even mention Hill, who would immediately be the best back-up SF of this era. He would have a sizeable positive impact on the team as well.