Mateo will start at shortstop and bat sixth in Wednesday's game against the Cardinals, Roch Kubatko of MASNSports.com reports.
Originally scheduled to receive Wednesday's game off, Mateo will enter the starting nine after the Orioles scratched Ryan Mountcastle (undisclosed). Mateo will put a six-game hitting streak on the lineup in the penultimate game of the series in St. Louis.