Rondo hasn't been the healthiest guy lately, and i don't think his biggest weakness (shooting) will go away. he may improve a little but who goes from horrible to good? it's very very rare. it also may be physical (huge hands, he's like a 7 footer).
While Westbrook has amazing upside, his stock seemed down. He could catch up with Rose. Physically, he looks like he could be as good a defender as Rondo someday.
And, Celtics could really use some penetration.
Not to mention, they could really use Perkins.
It's an awesome deal for Celtics. But if they did make that call, I doubt they actually expected a yes. It's too good.