Manager Bob Melvin said Mengden (ribs) has started a rehab assignment at Oakland's spring training complex in Arizona, Susan Slusser of the San Francisco Chronicle reports.
Mengden made his return to action with the team's rookie-league affiliate Friday night, tossing two scoreless innings while racking up three strikeouts on 81 pitches. Despite the positive step in his recovery, the 24-year-old still has a ways to go before he's able to return to regular action with Triple-A Nashville, as he hadn't pitched since tossing 4.2 innings in a start for the A's on June 3.