The 23-year-old right-handed starter out of Vanderbilt was scheduled to start for Triple-A Sacramento Tuesday, but was scratched.
He's a starting pitcher, but the A's don't need him in that role, so it's likely that he will pitch out of the bullpen. The roster spot that Gray will take isn't immediately evident.
Gray is 8-5 with a 2.81 ERA and 107 strikeouts for the River Cats.
Talking about Gray last week, general manager Billy Beane said the club was in no hurry to rush him, but that by the way he was pitching, Gray was getting closer to being big-league ready all the time.