McMahon will start at second base and bat seventh Thursday against the Marlins, Patrick Saunders of The Denver Post reports.
Garrett Hampson was the presumptive favorite at second base entering camp, but McMahon wrestled the primary job away by slashing .424/.470/.763 during the exhibition slate. McMahon could start all four games in the series with three other right-handers (Trevor Richards, Pablo Lopez, Sandy Alcantara) set to follow Jose Urena for Miami.