Manager Dave Martinez said Friday that Gallo's MRI revealed a "significant hamstring strain," Bobby Blanco of MASNSports.com reports.
Martinez didn't give a return timeline for Gallo, simply saying that the 30-year-old will be out for "a while." Joey Meneses has been acting as Washington's primary first baseman since Gallo landed on the injured list Wednesday, but it's possible the Nationals decide to add someone from their farm system or outside the organization to help split the workload.