Johnson specifically mentioned that he would like to see Mattheus pitch multiple innings in one outing.
"That's where he gets stretched out and where he needs to be," Johnson said.
• Entering Friday night's game, the Nationals were one of two teams in the Major Leagues with a winning record (48-47) and negative run differential (-14), joining the Yankees (51-44, -2).
Tom Schad is an associate reporter for MLB.com. Follow him on Twitter @Tom_Schad. This story was not subject to the approval of Major League Baseball or its clubs.