Just four days ago, Bob Melvin deemed it a "long shot" that left-handed pitcher Sean Manaea would return this season.
On Monday, the A's all but confirmed that they've lost the 26-year-old for the year.
Manaea will undergo arthroscopic surgery on his left shoulder on Sept. 19. No timetable for his return was given by the A's.
The A's placed Manaea on the disabled list on August 26 with left shoulder tendinitis. In 27 starts this season, he posted a 3.59 ERA with 108 strikeouts in 160.2 innings.
Manaea is the latest A's pitcher to undergo surgery on his throwing arm, joining RHP Jharel Cotton, Daniel Gossett and Kendall Graveman.
At 87-57 entering Tuesday's game in Baltimore, the A's currently hold the second AL Wild Card spot.