Here's the way I look at it:
How many world class athletes that are 6-4 make it to the NBA?
The answer? Not many. In fact, darn few.
Based on that alone, the chance of ANYONE, be it Michael Phelps or Jason Kidd, making it to the NBA is remote at best. Given that fact, when you ask about one particular one, the answer is always going to be the same: not likely. It is rare enough that a random BASKETBALL player makes it to the NBA.
In my mind, this question is pointless, since Phelps has shown no proficiency at basketball. Whether he COULD or not isn't a question; the fact remains that he HASN'T.
Which, of course, could make him a top level draft choice for Michael Jordan and the Bobcats. (It's all about POTENTIAL, you know.)