Hernandez, 23, drew Oakland's interest with a dominant performance in the 2007 Arizona Fall League as a member of the Phoenix Desert Dogs, the same team that featured a number of A's prospects.
He made 12 appearances without allowing an earned run for the AFL champions, and when the White Sox left him unprotected for December's Rule 5 Draft, Oakland selected him in the Major League phase.
The A's paid Chicago $50,000 to nab Hernandez, who had spent five years in the White Sox farm system, and the right-hander had to remain on Oakland's 25-man roster for the entire 2008 season or be offered back to his original organization, unless he were traded or cleared waivers.
Hernandez had a solid spring, posting a 2.70 ERA in eight appearances, but after picking up his first Major League victory last Monday in Toronto, he surrendered six runs on four hits and three walks over his next two outings before being designated for assignment.
The White Sox had the option to allow Oakland to keep Hernandez, who would have been sent to Triple-A Sacramento were that the case, but Chicago took him back and, in accordance to the Rule 5 Draft rules, repaid the A's half of the money they spent to select him.
"He obviously showed enough in the [Arizona] Fall League last year to catch our eye," A's manager Bob Geren said Wednesday before the opener of a two-game series against the visiting Mariners. "And that's obviously the reason they would take him back."
Hernandez was replaced on the 25-man roster by veteran righty Kirk Saarloos, who threw two shutout innings in relief against the host White Sox on Tuesday afternoon.
Mychael Urban is a national writer for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.