Mateo (elbow) is starting at shortstop and batting seventh in Sunday's split-squad game against the Phillies, Jacob Calvin Meyer of The Baltimore Sun reports.
Mateo underwent UCL reconstruction surgery on his left (non-throwing) elbow in late August and will be making his first start of spring training. The 29-year-old hasn't officially been ruled out for Opening Day, but manager Brandon Hyde recently said Mateo is unlikely to be available.