It's definitely a late switch, considering the tournament is already under way in the Far East, but Steve Gilbert reports that David Hernandez was "ruled ineligible for Team Mexico." Jack Magruder then explained a little more:
@katiemarek ... To be eligible to play for a country, grandparents or parents must have been born in that country. His GPs were born in US.— Jack Magruder (@JackMagruder) March 2, 2013
That's odd, because just yesterday, Nick Piecoro described Hernandez as "a third-generation Mexican-American," even though he doesn't speak any real Spanish. However, later on, the same piece says that David will "be representing his great-grandparents’ native country." From what I can find, that doesn't jibe with the generally-accepted definition of "third-generation", but may have been in part what led to the confusion. Still, you would think it's something which might have been definitively decided before the games began?
Now, it has certainly been the case that WBC qualifications are kinda loose. For instance, when Israel were attempting to qualify (they lost to Spain), only three of their 28 players were from the country. According to the New York Times., "Players do not need to be citizens of a participating country; they merely need to be eligible to become one. For Israel, that includes anyone with a Jewish parent or grandparent. A non-Jew married to a Jew could also play." That led the secretary-general of the Israel Association of Baseball to bemoan, tongue probably slightly in cheek, "If we only could get a nice Jewish girl to marry Albert Pujols..."
Hernandez will be replacing Indians closer Chris Perez on the roster: Perez has been out for 3-4 weeks with a sore shoulder. It puts Hernandez alongside bullpen colleague Heath Bell as an American reliever - the team also includes Willie Bloomquist. It's safe to say reaction to the move has been...mixed.
Which is pretty unfair. Hernandez didn't actually jump ship: he was ruled ineligible to play. But it does seem...well, let's go with "kinda convenient", that the decision on his eligibility happened at a perfect time, exactly the moment a slot opened up on the American roster. If that hadn't happened, would anything have been said? [Pauses to adjust tin-foil hat] Still, as a non-Mexican, non-American, D-backs fan, it doesn't make any real difference to me. I'll still be cheering him on regardless.