Gray remained in the hospital overnight with severe gastritis and was expected to be released some time Wednesday.
The condition involves inflammation in the lining of the stomach. Oakland team physician Dr. Allan Pont said of all the cases of severe gastritis he's seen in 32 years with the A's, this was the first requiring a hospital visit.
Gray is second in the AL with a 2.09 ERA and 9-3 for the last-place A's. He was scratched from his scheduled start on Tuesday night because of the ailment. A's manager Bob Melvin does not know when his ace will be able to pitch again.
The A's is hoping he can start Sunday against the Seattle Mariners.