A's left fielder Yoenis Cespedes left Sunday's game after the third inning with a sprained left thumb.
He was replaced by Jonny Gomes, and there was no immediate word on the severity of the injury.
Cespedes missed 22 games in May with a strained muscle in his left hand. He also sat out nine games in June with a hamstring strain but avoided the disabled list.
The A's cleanup man is hitting .260 and his nine homers and 35 RBI rank third on the team.
The A's and Mariners were tied 1-1 in the bottom of the sixth.