Gray left Saturday's game against the Cubs with right extensor muscle soreness, Susan Slusser of the San Francisco Chronicle reports.
Gray held the Cubs to two runs over five innings without a walk before being forced off the hill (72 pitches). He was replaced on the mound by Liam Hendriks. While the A's did not specify which extensor muscle was bothering Gray, the team did mention that the injury was to his right side, so it's presumably an issue with his pitching forearm. His status for his next start is up in the air.