Oakland dealt outfield prospect Jameson Hannah to Cincinnati. The Reds are sending Oakland $2.1 million to offset much of the $3,225,806 remaining in Roark's $10 million salary.
The trade was announced Wednesday, about an hour before the deadline for making deals. The A's began the day a half-game out of the second AL wild-card spot.
Roark lasted a season-low 3 1/3 innings during Cincinnati's 11-4 loss to the Pirates on Tuesday. He is 6-7 in 21 starts with a 4.24 ERA and can become a free agent after the season.
The Reds traded Roark a few hours after getting Trevor Bauer from the Indians for right fielder Yaisel Puig in a three-team swap that included San Diego.
Manaea struck out eight in six innings of a rehab start at Triple-A this week. He's likely to get one more rehab start before being activated. He had shoulder surgery last September.
Hannah, a 21-year-old center fielder, is hitting .283 with 31 RBIs and a .723 OPS in 92 games for Class-A Stockton in the California League. He was a second-round pick by the A's in the 2018 MLB draft.
The Associated Press contributed to this report.
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