Mengden will be recalled from Triple-A to start Wednesday's game against the Cardinals, Susan Slusser of the San Francisco Chronicle reports.
The 26-year-old righty has a 3.47 ERA, 1.11 WHIP and 50 strikeouts in 49.1 innings at Triple-A this season, but fared much worse (5.09 ERA, 1.65 WHIP, 19 strikeouts) in 23 MLB innings earlier in the year. Even so, he should be given a bit of a leash this time around, given Oakland's depleted big-league rotation.