The team made the announcement Thursday, and general manager David Forst said there's no timetable for the right-hander beyond the preliminary stretch.
This is a big blow to the A's and Gray, who was eager to start fresh following a forgettable 2016. He was scratched from his scheduled outing Opening Day last year because of food poisoning and never had the kind of season he envisioned as the leader of the rotation.
He spent two stints on the DL and went 5-11 with a 5.69 ERA on the heels of back-to-back 14-win campaigns. Gray, who received a $3,575,000, one-year contract to avoid arbitration, initially had plans to pitch for the U.S. in the World Baseball Classic.