This rumor doesn't make any sense, in my mind.
(1) The Cs lose financial flexibility, which I assume would be a major reason for considering trading Rondo (flexibility is lost because Gay is due an extension next year, just like Rondo).
(2) Cs get an upgrade in bench help in Gay, but a downgrade at point in Conley. Yes yes, I've seen the stats, but Conley has proven nothing in the league, whereas Rondo has proven he can effectively start on a championship team (at the very least he has proven that...).
(3) Huge risk for the Cs, given that the Big Three still have a window, but might not be as effective 2-3 years from now when Conley and Gay can really contribute (that's right, I don't think either can contribute in a big way this year -- see (2) above).
(4) Dropping Gay from the deal and getting the #2 pick instead changes my thinking. I would seriously consider such a deal, but only if there was a follow up deal to get a legit starting PG, at least for a couple years while Rubio and Conley battle it out and "earn their chops".